Stoichiometry of litter decomposition under the effects of global change: A meta-analysis.

doi:10.21203/rs.3.rs-2579152/v1

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Stoichiometry of litter decomposition under the effects of global change: A meta-analysis.

https://doi.org/10.21203/rs.3.rs-2579152/v1

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The cycling of nutrients from plant litter has relevant implications on the functioning of terrestrial ecosystems by controlling the availability of nutrients and net primary production. The effects of global change have been widely studied for most aspects of ecosystem functioning, but the direct implications on the stoichiometry and nutrient dynamics of litter decomposition are still poorly understood. We conducted a meta-analysis to determine how warming, changes of water availability, and enrichments of N and P interfere with the immobilisation/mineralisation of nutrients and the stoichiometric relationships during litter decomposition. Our database consisted of 194 experiments from 43 studies that experimentally simulated (i) warming, (ii) drought, (iii) irrigation, (iv) N enrichment, (v) P enrichment, and (vi) combined N and P (N + P) enrichment. We compared the early stages (approximately half a year) and late stages (approximately one year) of decomposition; and the specific effects taking into account the climate, the species used and the ecosystem type. We explored the different effects of all treatments and highlight three of them. (i) Warming and nutrient enrichment induce a faster release of C during decomposition, prominent in grassland and coniferous forests of continental climates, and with a potential feedback loop to climate change. (ii) C:P and overall C:N ratios generally decreased in most of the scenarios of global change analyzed at short- and long-term during litter decomposition, while the N:P ratios are more resilient to change. (iii) P limitation might be exacerbated in in warming continental climates; arid environments experiencing droughts; temperate environments with increasing water availability; and temperate broadleaved forest experiencing N and P enrichment. Our results provide information about the fate of litter decomposition and its nutrient and stoichiometric dynamics in response to drivers of global change. However, further experimentation and analysis considering all interacting drivers are warranted.

Litter decomposition

stoichiometry

global change

meta-analysis

carbon release

C:N:P

Nutrient stoichiometry is an important factor in the control of important ecological processes, from the growth rate of organisms and the style of life to ecosystem diversity, structure, and functionality (Sterner and Elser 2002; Sardans et al. 2012b; Peñuelas et al. 2019). The decomposition of litter is one of the key processes in the functioning of ecosystems (Zhang et al. 2018; Lanuza et al. 2019). In the current scenario of global change, the biosphere is not only facing changes associated with climate such as global warming or the perturbation of precipitation regimens (IPCC 2021), but also other types of changes such as alterations in the availabilities of elements, e.g. C or N, which are increasing in ecosystems due to anthropogenic activity, but the availabilities of other earth-bound nutrients such as P and K are not increasing to the same extent (Sardans et al. 2012a, 2021; Peñuelas et al. 2020). These stoichiometric changes and imbalances are shifting ecosystem elementomes (Peñuelas et al. 2019; Fernández-Martínez 2022; Peñuelas and Sardans 2022). This concept is understood as the content of all or at least most of the elements in, for this case, soil and litter. The shifting elementome and the effects of climate change or the loss of biodiversity may produce a different dynamic for processes such as litter decomposition and the general patterns of C and nutrient cycling in plant-soil systems.

Litter is an important source of mineralisable nutrients for microbial and plant metabolism in terrestrial ecosystems (Silveira et al. 2011), and its decomposition is a fundamental process controlling the availability of nutrients in soils, the cation-exchange capacity, soil pH, and the formation of hummus (Berg and McClaugherty 2008). The release of nutrients allows microbes and plants to take them up, but litter decomposition is also important at a biospheric level for several reasons, such as its role in the exchange of greenhouse gases and its affect on C stocks in soil (Walkiewicz et al. 2021) that may in turn affect climate change (Parker et al. 2018). Litter decomposition is affected by drivers of global change such as increasing temperatures and precipitation that can then increase the rate of decomposition (Zhang et al. 2008), droughts that slow decomposition and increase litter C content (Deng et al. 2021), changes in UV radiation (Yan et al. 2021), and nutrient enrichment, having diverse responses to N enrichment across ecosystems: increasing the rates of decomposition in grasslands and wetlands and decreasing it in forests (Su et al. 2022). These drivers of global change thus distinctively affect the rates of litter decomposition and nutrient mineralisation and will also determine the shifting of elementomes in soils.

Litter decomposition has been a widely studied filed, even in relation with global change analyses (Zhang et al. 2008; Deng et al. 2021; Su et al. 2022), but there is la lack of understanding on how litter stoichiometry during decomposition process vary in relation with the world drivers of global change. When looking into litter dynamics, most review studies focus on one aspect such litter decomposition rate (Zhang et al., 2008; Su et al., 2022) or the effect on one elemental content (Deng et al., 2021) or did not performed an exhaustive quantitative analysis of the effect sizes (Sardans et al. 2012a). Our aim with this analysis is to compile experiments that recorded the stoichiometric relationships of at least two elements at the same time (C, N, P, or K) in decomposing litter, and that simulated one of the following drivers of global change: (i) warming, (ii) drought, (iii), increase in water availability, hereafter irrigation, and (iv) the enrichment of N and P (separately or combined). We compared nutrient dynamics in early (approximately six months) and late (approximately one year) stages of decomposition and also took into consideration factors such us climate, species type and ecosystem type. Restricting to studies that portrayed the simultaneous response of two elements to global change reduced the number of studies available for meta-analysis, but allow as to look at the elemental ratios. Moreover, we also looked at the effects over decomposition rate and individual C,N,P and K contents and concentrations.

Based on a previous review of C:N:P stoichiometry of ecosystems under global change (Sardans et al., 2012a), we expect that warming reduces C:N in non-limited water environments. We also expect that the N enrichment decrease C:N and increase N:P ratios. We hypostatize that N + P enrichments treatments reduce C:N and C:P ratios; and P enrichment alone reduce both N:P and C:P ratios. Irrigation treatments may increase litter decomposition rates and thus C mineralization. Hence, C:P and C:N ratios may reduce. N:P ratios may be reduced also due to the lower solubility of P. Regarding the previous meta-analysis (Zhang et al. 2008; Deng et al. 2021; Su et al. 2022) we expect similar results: litter decomposition increasing in warming and irrigation treatments; drought reducing litter decomposition rate and C content; and N enrichment reducing litter decomposition rates in forest but increasing in grasslands.

2.1. Data collection

To compile a data set of the effects of the drivers of global change on litter stoichiometry, we conducted a bibliographic search on 1st of February 2022 for peer-reviewed papers on the public databases Scopus, Web of Science, and Google Scholar. Three searches were performed: (i) (litter AND decomposition) AND (warming OR (increase* AND temperature*)) AND experiment* AND ((N:P) OR (C:N) OR (C:P)) for global warming as an agent of change; (ii) (litter AND decomposition) AND ((N OR P) AND (deposit* OR fertiliz* OR addition)) AND experiment* AND ((N:P) OR (C:N) OR (C:P)) for nutrient enrichment (e.g.,fertilisation or enhanced deposition) as an agent of change, and (iii) (litter AND decomposition) AND (precipitation OR rainfall OR drought) AND (reduc* OR addition OR increment* OR exclusion) AND experiment* AND ((N:P) OR (C:N) OR (C:P)) for changes in the availability of precipitation as an agent of global change. After screening the manuscripts, we selected studies that reported: (i) plots with experimental and control conditions, (ii) measurements of the variables in the experimental and control plots that were conducted under the same temporal and spatial scales, and both the magnitude of the treatment and experimental duration were clearly recorded and (iii) enough data to calculate the C:N, C:P, and/or N:P ratios of the remaining litter. Our final database contained 43 studies (Table S1; Aerts et al., 2001, 2012; Allison et al., 2013; Andresen et al., 2010; Blok et al., 2016; Cornejo et al., 1994; Day et al., 2008; Fang et al., 2021; Gong et al., 2020, 2015; Gossiaux et al., 2020; Hong et al., 2021; Jiang et al., 2018; Jonasson et al., 2004; Li et al., 2019; Longhini et al., 2021; Madeira et al., 1995; Mei et al., 2022; Niu et al., 2010; Ribeiro et al., 2002; Rustad and Fernandez, 1998; Sanaullah et al., 2014; Saura-Mas et al., 2012; Shi et al., 2021; Song et al., 2017; Su et al., 2021a, b; Tie et al., 2020; Vallicrosa et al., 2021; Van Meeteren et al., 2007; Wang et al., 2017, 2018; Xie et al., 2020; Xu et al., 2012; Yahdjian et al., 2006; Yavitt et al., 2004; Yoshitake et al., 2020; Zheng et al., 2017; Zhou et al., 2018; Xiao et al., 2021). All data for each study were extracted from the associated published data sets provided by the author/s or were extracted from figures using WebPlotDigitizer (Rohatgi 2020). We considered independent experiments (different studies for meta-analysis) from these 43 studies when (i) the litter species differed, (ii) plots differed geographically within the same study, (iii) different treatments were applied independently, and (iv) plots differed in the intensity of treatment. After applying these requirements our database contained 194 experiments.

We recorded ancillary data for each study, including information for the study site: geographic location of the sampling sites (latitude, longitude, and elevation), climate (mean annual temperature and precipitation), and ecosystem type (forest, shrubland, grassland, or others). We also complied information for the litter species sampled, the method of litter sampling (litterbag experiments vs litter collection), and initial litter biomass and nutrient concentrations. The experimental conditions were characterised in three groups: (i) increase in temperature, the intensity of which was recorded in °C, (ii) enrichment of N, P or both N and P, the intensity of which was recorded as g m^− 2 y^− 1, and (iii) changes in the amount of precipitation, which was recorded as the percent decrease or increase in water availability relative to the control plots in a year. We ultimately analysed six data sets: one for warming treatments (47 experiments), one for enrichment with N only (55 experiments), one for enrichment with P only (13 experiments), one for enrichment with both N and P (38 experiments), one for decreases in the amount of precipitation (drought treatment, 13 experiments), and one for increases in the amount of precipitation (irrigation treatment, 12 experiments). The starting dates of the treatments and the litterbag experiments were compiled with the associated duration (in days) of the litterbag experiments, which was the case for 26 of the 43 studies (and a corresponding number of 123 experiments). We then compiled both the control and experimental biomasses remaining in percentages (only for the litterbag experiments), the nutrient concentrations, nutrient contents (percentages of the initial nutrient contents remaining) C, N, P, and K; and the C:N, C:P and C:K ratios of the litters. The standard deviation and number of replicates for all these variables for both the control and treatment plots were also recorded.

2.2. Calculation of variables

Depending on the study, the nutrient data were the concentration of the nutrient, the content that remained relative to the initial value, and/or the ratios of the elements. We calculated the concentrations of C, N, P, and K, the contents of C, N, P, and K remaining relative to the initial values, and the C:N, C:P, and N:P ratios (on a mass basis, for the mass remaining) for all experiments when possible. We initially recorded all sampling dates for each experiment, but only end up using one. We selected the sampling dates closer to 180 d. This database had a median of 233 ± 26 d, so it represented dates closer to half a year, or “early” stage of decomposition (see below). We also wanted to explore the effects of global change at later stages of decomposition. We subsetted then for sampling dates closer to 1080 d. This database had a median of 412 ± 32 d, so it represented dates closer to a year, or “late” stage of decomposition. Hence, we had a database for all experiments at an “early” stage of decomposition, and other and a “late” stage. We also calculated the decomposition rate based on the classical definition (Olson, 1963):

$$k=-\frac{\text{ln}{X}_{t}/{X}_{0}}{t}$$

where $k$is the decomposition rate, $t$is the duration (in days) of the experiment, and ${X}_{0}$ and ${X}_{t}$ are the initial litter mass and the litter mass remaining at the time of sampling, respectively (percentage of the initial mass). We also applied this formula to the initial and remaining litter masses, the elemental contents, the initial and remaining litter elemental contents (for obtaining the rates of release of the elements), and to the initial and remaining litter elemental ratios (for obtaining the rates of change of the elemental ratios).

We assessed the effects of the treatments on the mass remaining at the end of the experiment and the decomposition rates of the litter biomasses before determining the stoichiometric effects of these drivers of global change. We determined the effects of the treatments on the litter concentrations of C, N, P, and K and calculated the nutrient contents remaining (% from the initial) after the experiment (when the remaining contents were not provided). The initial litter masses and nutrient concentrations were thus needed, so the number of experiments used for the analysis of content was lower than for the concentrations. We also determined k for the contents remaining (rate of release), but the number of experiments (> 5) for the remaining K contents was insufficient and were excluded from the analysis. The effect of global change on K contents is not addressed in this study, but we addressed the effect over K concentrations in litter. Ratios were calculated as the quotient of the nutrient contents, but we used the ratios provided when these data were not available. To calculate k (rate of change) associated with the shifting ratios, we needed the initial ratios at the start of the decomposition experiments.

2.3 The final data set

Our final data set contained 42 sampling locations (Fig. 1) and data for litter from 71 species. We distinguished 194 experiments within the studies selected. Based on the Köppen-Geiger classification (Peel et al. 2007), 3.6% of the experiments were carried out in tropical biomes, 24.7% in arid biomes, 34.0% in temperate ecosystems, 19.1% in continental ecosystems, and 16.5% in polar ecosystems (the remaining 2.1% were from experiments without climatic classification data). The starting months for the litterbag experiments differed across studies: 11% started from January to March, 25% from April to June, 6% from July to September, and about 15% from October to December. Figure 1. Global map of the sampling locations (pink dots) used in the meta-analysis.

We used 178 of the 194 experiments, which were those that only entailed one treatment (warming, drought, irrigation, N enrichment, P enrichment, or N + P enrichment). We calculated the effects of our chosen treatments for the decomposition of biomass and for the nutrient concentrations, contents, ratios, and decomposition rates

2.4 Statistical analysis

We used the metafor package (v2.4-0; Viechtbauer, 2010) in R Studio (v4.1.2; R Core Team, 2021) for the statistical analyses. We calculated the sizes of the effects of each treatment (warming, N enrichment, P enrichment, N + P enrichment, increase in water availability, and decrease in water availability) on the remaining litter biomass, the initial concentrations of C, N, P, and K, the contents of C, N, P, and K remaining, the decomposition rates (k) of these contents, the C:N, C:P, and N:P ratios, and the decomposition rates (k) of these ratios. To calculate effect size of a treatment, we calculated the standard mean difference (SMD), where the numerator of SMD was the value of the variable under the treatment minus the value of the variable under the control (retaining negative signs). A positive treatment effect (SMD) thus indicated that both variables either increased or decreased together, and a negative treatment effect indicated that one variable increased as the other decreased. We constructed meta-analytic models as random-effect models using Restricted Maximum Likelihood method (REML; Cook, 2014) with the rma.mv function. We rejected the meta-analytic models when the number of experiments was < 5. We calculated the general effects of the treatments over the experiments using the study as the random factor, to account the possible non-independence of experiments from the same study. For the treatments that significantly affected the variables, we calculated the sensitivity of the effect size to the intensity of the treatment by including the intensity as a moderator. We also explored the effects of the treatments over the experiments taking account of factors such as climate (arid, continental, temperate, polar or tropical; according to Köppen-Geiger classification); ecosystem type (grassland, forest or shrubland) and specie type (broadleaved tree, conifer tree, grass, shrub). We added the factor (climate, ecosystem type or species type) as a moderator of the random model, with study as the random factor. In the case of species type, the random factor was specie nested within study. For those analysis, we only used data from the early decomposition database and of the effect of decomposition rate, C,N and P contents, K concentration and C:P, C:N and N:P ratios.

In order to summarize the results, we constructed a summary table were we explore the possible consequences of global change over decomposition rates and C, N, P and K concentrations, contents and ratios of the remaining litter. Those scenarios are the result of mixing the different treatments. The effects are measured over decomposition rate (from the effects of decomposition rates itself and the effects for litter mass remaining); C, N, P and K presence in litter (from the effect over elemental contents, concentrations and element release rates); and C:N, C:P and N:P ratios (from the effect over the ratios and the ratios change rate).

3.1 General effects

We explored the general effects of the treatments over the experiments. The detailed data of the significant effects founded in the general effect analysis are present in Table 1

Litter decomposition

The amount of mass remaining was lower in N + P enriched plots than in controls in both early (6 months) and late stages (12 months), but the effect over litter decomposition rates was only found for late stages. Plots under irrigation treatment presented higher decomposition rates than control plots in late stages of decomposition (Fig. 2). When analysing the sensitivity of the effects to the treatment intensity, we found that in the case of N + P enrichment experiments, the more N is added the higher is the effect, but this was only found for late stages. Warming, drought and N or P enrichment (alone) did not shown any effects for litter mass decomposition.

Effects on the nutrient concentrations

C concentration in litter decreased, compared to control plots, under P enrichment treatment in both early and late stages of litter decomposition, and also under simulated drought in early stages (Fig. 3). The sensitivity analysis revealed that the more P was added, the higher the effect was for both early and late stages. N concentrations in litter increased, compared to control plots, under P enrichment treatment in late stages of decomposition, and decreased, also in late stages, under drought treatment (Fig. 3). The sensitivity analysis revealed that the more P was added, the higher the effect was over N concentrations. P concentrations in litter increased under N + P enriched treatments in both early and late stages of decomposition, and decreased under drought treatment (Fig. 3). The sensitivity analysis revealed that the more P was added (in the N + P enrichment treatment), the effect was in the late stage of decomposition. K concentrations in litter decreased in warmed plots compared to control plots in late stages of decomposition (Fig. 3). The sensitivity analysis revealed a significant relationship between the intensity of the warming and the decreasing effect of K concentrations.

3.3. Effects on the nutrient contents remaining.

C content in litter decreased, compared to control plots, in warmed plots in early stages in decomposition. Warming also increased the C release rate of litter in late stages of decomposition. C content in litter also decreased in N + P enriched plots in late stages of decomposition. N content in litter was incremented in late stages of decomposition under N + P enrichment conditions. Any effect was found for N release rate of P content or release rate (Fig. 4).

3.4. Effect on the nutrient ratios

Litter C:N ratios decreased, compared to control plots, in P enriched plots in both early and late stages of decomposition, and with N enrichment in early stage of decomposition. C:N change rate increased under N enrichment in late stages of decomposition and under N + P enrichment in both stages of decomposition (Fig. 5). The sensitivity analysis revealed that the more P was accounted in the P enrichment experiments, the higher the effect is over the C:N ratio. Same happened with N enrichment intensity in the N enrichment experiments, the more N was accounted, the higher the effect was over the C:N ratio. Litter C:P ratio decreased, compared to control plots, under warming treatment in both early and late stages of decomposition. Warming also increased in early and late stages the C:N ratio change rate. N + P enrichment treatment decreased C:P ratios in the late stage of decomposition, while C:P change rate increased also in late stage of decomposition (Fig. 5). The sensitivity analysis revealed an indirect relationship between intensity of the warming and the decreasing effect of C:P ratio. Litter N:P ratio decreased, compared to control plots, with the irrigation treatment for the early stage of decomposition, and no other effects were found for N:P ratios of N:P change rates (Fig. 5).

3.2 Effects considering the climate of the sampling area

We explored the effects of the climate over the effects of the global change experiments. The significant effects are summarized in Table 2.

We found a general decreasing effect of C contents in litter remaining in warming experiments (Table 1). The experiments from continental climates were the ones that had the C content decreasing effect. Regarding N content, we found that although we did not find general significant effects, we found a N content decreasing effect in irrigation experiments in continental climates. We also found an increasing N content effect in drought experiments in arid climates. Although we did not find any general effect for P contents, we did taking into account the climate. Under warming experiments, we found increasing P content effect in continental climates, and decreasing in polar climates. In drought experiments we found an increasing P content effect in arid climate. In irrigation experiments we found an increasing P content effect in temperate climate. Last, in N + P enrichment experiments, we found an increasing P content effect in temperate climate and a decreasing P content effect on arid climate. Despite not finding general significant effects for K concentrations, we found a decreasing K concentration effect on irrigation experiment in temperate climate. Also, we found a decreasing K concentration effect in warming experiment in continental climate, and a increasing one in temperate climate.

C:N ratio was also affected by climate in warming and N + P enrichment experiments despite not having any general significant effect. We found a C:N ratio decreasing effect in warming experiment on continental climate, Also, in N + P enrichment experiments we found decreasing C:N effects for polar and temperate climates. In N enrichment experiments, which had a C:N ratio general decreasing effect, climate related effects were found. C:N ratio decreasing effects were found in continental and tropical climates. C:P ratio was affected by climate in N + P and N enrichment experiments although no general effects were found. We found, for N + P enrichment experiments, a C:P ratio decreasing effect in continental and tropical climate. We found also a C:P decreasing effect in N enrichment experiments in continental climate. For warming experiments, which showed a general decreasing effect of C:P ratios, we found that continental climate had the C:P ratio decreasing effect.

3.3 Effects considering the species type and ecosystem type of the sampling area

We explored the effects of the species and ecosystem types over the effects of the global change experiments. The significant effects are summarized in Table 3 for species type and Table 4 for ecosystem type.

Although there was no general significant effect for litter decomposition rates, we found that grass species had an increasing effect over litter decomposition rates in warming experiments. Also, in N + P enrichment experiments, conifer trees had an increasing decomposition rate effect.

We found a general decreasing effect of C contents in warming experiments (Table 1). The experiments with grass species or grassland ecosystems had the C content in litter remaining decreasing effect. Although there was not a general effect for C content on drought experiments, we found a decreasing C content effect of shrub species. Despite not finding any general effect for N content in irrigation experiments, we found that grass species had a decreasing N content effect. P content did not have any general effect for N + P enrichment experiments, but we found that broadleaved tree had an increasing P content effect. Under the same treatment, we found increasing P content effect in forest ecosystem and a decreasing effect on grasslands. Despite not finding general significant effects for K concentrations, we found a decreasing K concentration effect of shrub species on N enrichment experiments. Also, in warming experiments, we found a decreasing K concentration effect in forest ecosystem and an increasing effect on shrubland.

C:N ratio was affected by warming, drought and N + P enrichment experiments although not having any general effect. Warming showed a decreasing C:N ratio effect in grass species. Drought showed an increasing C:N ratio for shrub species and shrubland ecosystem and a decreasing effect for grass species and grassland ecosystem. N + P enrichment showed a decrease of C:N ratios for conifer trees and forest ecosystem. In N enrichment experiments, which had a C:N ratio general decreasing effect, we found that forest ecosystem were the ones having the decreasing effect. Although not showing any general effect for N + P enrichment experiments, C:P ratio in forest ecosystems had a decreasing C:P ratio effect. For warming experiments, which showed a general decreasing effect of C:P ratios, we found that grass species and conifer trees; and forest ecosystems and grasslands had a decreasing C:P ratio effect. N:P ratio showed a general effect decreasing effect for irrigation experiments (Table 1). When taking into account specie and ecosystem type, we found that broadleaved trees and forest ecosystems had the only N:P ratio decreasing effect.

3.4. Summary of the results

In order to facilitate the comprehension of the analysis, we constructed a summary of the results from the general effect analysis grouped by the distinct scenarios that the global change experiments wanted to portray (Fig. 6).

Our meta-analysis provides an overview of the dynamics and stoichiometry of litter decomposition under drivers of global change such us temperature, changes in water availability, and nutrient enrichment and limitation. The results differed from our initial expectations but portrait an important guide of the C:N:P stoichiometry under the fate of global change.

Effect of warming.

Our results showed, as expected, that C:N decreased only in experiments on continental climates (non water limited environments), the decreasing effect of C:N ratios were only observed in grass species. According to Köppen-Geiger classification, continental climates are characterized by cold temperatures and precipitations exceeding evaporation (Peel et al. 2007). The effect on C:P ratios, which had a general decrease, were also driven by experiments on continental climates, experiments from grass or conifer tree species and by experiments on grassland and forests. Moreover, C content, which generally decreased on early stages of decomposition, showed the effect on continental climates, grass species and grassland environments. Meanwhile, P contents did not show a general effect, but increased in continental climates and decreased in polar ones. Grasslands and forest of conifer trees dominate this environment and this may explain the similar results, as studies in continental climates are likely to be from grassland or conifer forest. The absence of water limitation in continental climates may enhance the effect of warming over litter decomposition (Petraglia et al., 2019). Warming under this conditions may be decreasing the C content in litter by stimulating the decomposing activity and enhancing the release of C, but there were not apparent general or specific effects of warming over decomposition. On the other hand, we saw an increasing decomposition rate when looking on the grass species, which was also expected from other studies (Song et al., 2014). While C contents decrease in those experiments, P is increasing, explaining the decreasing C:P and C:N effect (no effects were found for N contents). Warming in continental climates, dominated by grasslands and conifer forest, is then releasing C and immobilizing P in litter. C may be released to the soil but also to the atmosphere as a product of respiration, and hence contribute to the greenhouse effects and consequently to more global warming. P-limitation problem is induced by the P immobilization in litter being not available as a resource for plants. Finally, the C:N and C:P ratios were consequently reduced in these scenarios, with potential impacts on soil trophic webs.

Effect of water availability

We did not find much general effects for changes in the water availability. As expected, irrigation treatment reduced the N:P ratio of litter which may be caused by the distinct solubility of P (Manzoni et al., 2010).This effect is only present in early stages of decomposition, which supports our explanation as later stages of decomposition had enough time to compensate the differences in nutrient solubility. Also, the effect is only significant in broadleaved trees and forest ecosystems. Although the composition of the senescenced leaves of grasslands may have lower N concentrations than broadleaved forest, the P concentration tends to be higher (Yuan and Chen 2009; Vallicrosa et al. 2022). Forests of broadleaved species, with litters with higher N: P ratios, may thus be more sensible to changes in water availability than grasslands. Irrigation also increased the litter decomposition rates of late stages. Although we did not find general effects in irrigation treatments for N and P contents, we identified a P immobilization effect in temperate climates, and N release in experiments in continental climates. We expect that the hydrolysation of litter compounds such as cellulose, lignin, macroproteins, and DNA would be accelerated under greater availability of water. Unlike DNA, which is a P-rich compound, cellulose, lignin, and macroproteins are rich in C and N, and we would then expect a higher mineralisation rate for C and N than P. Drought immobilised N and P in litter only in experiments in arid climates. This biological immobilization of N and P could be related to an increase in nutrient use efficiency under water-limiting conditions (Sardans et al., 2012a). The effects of shrub species reducing the C content, but increasing C:N ratios in shrub and shrublands (without effects on the N content) seem contradictory, but this effect is driven by only one experiment. Similar contradictory results appear as C:N ratios were reduced in grass species and grasslands under drought, but also grass species showed N release effect while no effect for C contents. Again, the number of experiments was just one.

Effect of nutrient enrichment

We did not found any effect for litter decomposition under N enrichment, general or depending on the ecosystem, as expected by previous meta-analysis (Su et al., 2022). Nevertheless, we were focusing on studies with two or more elements sampled at the same time, and the number of observations for individual nutrient trends is thus lower than in those meta-analysis. We did find effects for increasing decomposition rates or litter mass remaining in N+P enrichment treatments for early and late stages of decomposition. While there was not general effect for litter decomposition rates at early stages (it was significant for litter mass remaining, and that is why is appears as an effect in Figure 6), we found that the decomposition rate increased in experiments using litter from conifer trees. A meta-analysis of nutrient enrichment in stream habitats (Ferreira et al., 2015) also reported that the litter decomposition rate increased only when both N and P were applied, suggesting a co-limitation of these elements to drive faster litter decomposition. High concentrations of N hamper the degradation of lignin and thus the rate of litter decomposition (Berg and McClaugherty, 2008), explaining the lack of general effect of N enrichment over decomposition. Thus, litter decomposition may be limited by P at high N:P ratios, driven by the enrichment of N alone and not combined with P (Güsewell and Gessner, 2009). Fast-growing decomposers, which generally dominate the decomposition of nutrient-rich organic matter (Fontaine et al., 2003), have high demands for P due to the high rates of cell division that entail the production of P-rich RNA (Elser et al., 2007, 2003). This would explain why the increasing decomposition rate in the early stages was only found in conifer litter, as conifer forests are less P-limited than broadleaved forest (Vallicrosa et al. 2022). In early stages of decomposition under N+P enrichment, P was immobilised in temperate broadleaved forest while released in arid grasslands. Temperate broadleaved forest have already high decomposition rates, an increase in nutrient supply would accelerate it more, maximizing the differences in mineralisation of C,N, and P. On arid grasslands under nutrient enrichment, which generally are lacking from nutrients, decomposers may be focusing on water use strategies that consume P, explaining the P release from litter. In late stages of decomposition under N+P enrichment, the increase in the decomposing activity was also reflected in the increasing release of C. N content, in the other hand, increased its immobilization in litter. The experiments of Güsewell (2019) found that N-limited decomposers in mesocosmic experiments immobilised N better than P-limited decomposers, but in contrast, N- and P-limited decomposers immobilised P with similar efficiencies. N+P-enriched experiments may not be N-limited, but they are more N-limited than N-enriched plots and this may explain the presented results.

As expected, N enrichment alone managed to generally reduce the C:N ratios in both early and late stages of decomposition, but any effect was found for the N:P ratios. The decreasing effect of the C:N ratios was driven by experiments on forest or in tropical and continental climates. When the enrichment was from N+P, there was also a general decreasing effect of C:N ratios, but it was driven by experiments in forest of conifer trees and in polar or temperate climates. N enrichment also decreased the C:P ratios of experiments in continental climates. The N+P enrichment treatments generally reduced C: P ratios in late stages of decomposition, in early stages, although a general effect was not found, we identified that there was a decreasing effect in experiments in tropical and continental climates, and in forest ecosystems. However, we identified that there was a decreasing effect in experiments in tropical and continental climates, and in forest ecosystems, were N, and mainly P are commonly limiting. In these cases, soil N and P enrichment could alleviate the need to mineralise nutrients from litter. The enrichment of P alone, which is a very uncommon scenario, it did not reduce the C:N and C:P ratios as we expected. Instead, it generally decreased the C:N ratio in early and late stages of decomposition. The stimulation on fast decomposers (highly dependent on P levels), may be behind this effects. But it does not explain the absence of effect on C:P ratios

K concentrations

This study has mostly focused on C:N:P stoichiometry due to the few experiments that provided data for K contents. The changes in the concentrations are not as good indicators of release/mineralization as contents, but may provide hints of the K decomposition dynamics. As an example, in the late stages of decomposition under warming K concentrations decreased as a general effect. K leaches faster than other elements such as N or P (Sardans and Peñuelas 2015) and thus is in not rare that its concentration decreased. But in early stages, there was not a general effect: we found that the concentrations increased in temperate climates and shrublands; and decreased in continental climates and forest. This increasing effect was only found in one experiment (Saura-Mas et al. 2012) which was carried out in a temperate shrubland while the other were carried out in continental climates (and less water limited than in that case), but portraits an interesting reflexion. K is an important element that improves water (Xu et al. 2021). In environments like the temperate shrubland, where drought stress is apparent, K concentrations increases in litter during warming experiments. Also in temperate climates under irrigation treatments K concentrations decreased, indicating that water availability plays an important role in the decomposition of K from the litter.

Limitations and implications of the study

Our initial question was about how drivers of global change interacted with the nutrient stoichiometry in leaf litter, so we only selected articles that sampled at least two nutrients simultaneously in litter samples for control and treated plots. The number of studies/experiments to test our hypothesis was thus reduced. We therefore used data from experiments of decomposition in litterbags but also from litter sampling without decomposition experiments, from which we could only extract data for the differences in nutrient concentrations or ratios between control and treated plots but not for the rates of mineralisation or decomposition. Caution is needed when interpreting these results, because elements such as K in treatments, similar to the decrease or increase in water availability, are sustained in fewer experiments. Furthermore, even though we screened studies from around the globe, the distribution of the sampling sites (Figure 1) was a clear example of the geographic bias frequently found in meta-analyses, where most of the field studies are within the latitudinal ranges of China, Europe, and North America. Another issue to consider is that the processes of decomposition vary across ecosystems, the chemical composition of litter, and stages of decomposition. Even though we compared experiments with similar durations (early or late stage of decomposition), the experiments in each study began in different seasons, which difficult the appropriate comparison of experiments. Our results can thus serve as general information for determining how nutrient dynamics may behave under our current scenario of global change and may differ across ecosystems and decomposing time. For further information about specific ecosystem dynamics, we detail the characteristics of each study used in Table S1. The use of k (rate of nutrient release and rate of change of nutrient ratios) was useful for identifying effects that were not apparent when focusing only on the nutrient contents in the litter. Finally, we focused on experiments and observations of one driving factor, but all drivers of global change interact in nature, so multifactorial experiments and observations are strongly recommended.

Our meta-analysis explored the effects of warming, changes in water availability, and nutrient enrichment, which are known drivers of global change, on the processes of nutrient mineralisation/immobilisation and on the stoichiometric relationships during the decomposition of litter. The effects can vary across scales, but they can help us determine the generalized effects of global change on the nutrient dynamics of terrestrial ecosystems.

We found that scenarios of warming and nutrient enrichment led to losses in litter C content. C can move from the litter to the soil but also to the atmosphere as CO₂, contributing to the greenhouse effect and generating a feedback loop, because more warming derived from the greenhouse effect may intensify the release of C from litter. Subsequently, the C: N and C: P ratios were reduced in these scenarios, with potential impacts on the soil trophic webs. This effect is prominent in grassland and coniferous forest in continental climates. In fact, in 78.3% and 58.7% of the studied scenarios of global change studied, C:N and C:P, respectively, decreased significantly during litter decomposition.

The N:P ratios were only affected by the increase in the availability of water in broadleaved forest, which may be caused by the higher solubility of N with respect to P. Our analysis found that N:P, in contrast with C:N and C:P ratios, ratios were more resilient to changes in temperature, water scarcity and nutrient enrichment.

We identified that P limitation problems, derived from the immobilisation of P in litter, may be found in warming continental climates; arid environments experiencing droughts; temperate environments with increasing water availability; and temperate broadleaved forests experiencing N and P enrichment.

This exploratory analysis is useful for distinguishing between some of the impacts of global change on ecosystem nutrient dynamics and should inspire new and warranted research on the unknown mechanisms responsible for our results and on interactions amongst multiple drivers. We encourage future research with experimental designs that simulate scenarios of global change for exploring the stoichiometric implications of the simulated scenarios with the maximum possible number of drivers interacting and in different stages of decomposition

Funding

This work was supported by the Spanish Ministry of Science and Digital Transition (TED2021-132627B-I00)

Competing Interest

The authors have no relevant financial or non-financial interests to disclose.

Author contributions

Javier de la Casa: Conceptualization, Data extraction, Statistical analysis, Writing. Jordi Sardans.: Conceptualization, Supervision, Reviewing and Editing. Marta Galindo-Díaz: Data Extraction. Josep Peñuelas: Conceptualization, Supervision, Reviewing and Editing.

Data availability

Data will be made available upon request.

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Stoichiometry of litter decomposition under the effects of global change: A meta-analysis.

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Abstract

Figures

1. Introduction

2. Materials And Methods

2.1. Data collection

2.2. Calculation of variables

2.3 The final data set

2.4 Statistical analysis

3. Results

3.1 General effects

3.3. Effects on the nutrient contents remaining.

3.4. Effect on the nutrient ratios

3.2 Effects considering the climate of the sampling area

3.3 Effects considering the species type and ecosystem type of the sampling area

3.4. Summary of the results

Discussion

Conclusion

Declarations

References

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