Effects of compost amendment
The addition of compost increased the levels of microbial and soluble C and total C in the surface soil. In contrast, below the surface, total organic C was depleted, although soluble and microbial biomass C increased. Thus, as suggested by other authors, higher soluble organic C inputs may have primed the decomposition of soil organic C (Liu et al., 2020). Higher levels of soluble C in the bottom soil were derived from adding compost to the soil surface and did not counterbalance the primed decomposition of organic C in that layer. Interestingly, primed decomposition of organic C in the organic C-depleted soil coincided with high increases in total Pi, likely due to leaching. Indeed, adding compost strongly contributed to increasing total Pi in both less labile and labile forms. Higher levels of NaHCO3-Pi in organic C-enriched soils are reported in the literature (Shen et al., 2014a) and have been related to a reduction in P sorption sites in the soil solid phase caused by an increase in soluble C (Guo et al., 2009). Thus, in our experiment, the increase in NaHCO3-Pi in the organic C-depleted layer may have partly arisen from the mobilization of retained P.
In agreement with our results, other studies report that the addition of organic fertilisers does not change total Po (Annaheim et al., 2015; Jantamenchai et al., 2022). As the total Po balance was not affected by adding compost, either in surface or subsurface soils, we suggest that the increases in EDTA-NaOH-Po and NaHCO3-Po in the organic C-depleted compost-amended soils may indicate an increase in mobilization of retained Po. Indeed, C-depleted soils showed two-fold increases in the NaHCO3 Po/Pi ratio after the addition of compost. Higher pools of NaHCO3-Po compared to NaHCO3-Pi have been described in soils low in organic C with a low or negative P balance (Romanyà and Rovira, 2007, 2009), suggesting Po mobilization occurs in C-depleted and low organic C soils. However, in our experiment, increased Po mobilization was also observed in organic C-enriched soils with four-fold increases in the NaHCO3 Po/Pi ratio and very large increases in NaHCO3-Po. This indicates that labile Po forms can be readily mobilized in C-enriched and C-depleted soils with higher levels of microbial biomass C and soluble C. Strong correlations between microbial (r = 0.918, p < 0.001) and NaHCO3-Po and between soluble C (r = 0.905, p < 0.001) and NaHCO3-Po suggest that Po mobilization may be promoted by the availability of soluble C to soil microbiota.
Long-term additions of compost have been found to significantly enhance Pi levels in soil with few effects on Po (Annaheim et al., 2015). This agrees with our results, as the largest increases in all Pi fractions and orthophosphate were observed in the organic C-enriched layer; they also increased in the subsurface layer, but to a lesser extent. Orthophosphate is generally the predominant form of Pi in soils, and mineral fertilisation has been shown to increase pools of orthophosphate (Wu et al., 2021) and Pi (Wu et al., 2022). In the present study, in agreement with other authors, we found that orthophosphate increased after the addition of organic fertilisers. Pools of pyrophosphate, which mainly originate from hydrolysis by microbial pyrophosphatases (Cade-Menum, 2015), were not affected by compost amendment despite the considerable increase in microbial biomass.
The pool of Po in EDTA-NaOH extracts was dominated by IHP. In contrast with other studies (Dou et al., 2009; Annaheim et al., 2015), we found significant increases in total IHP in both organic C-enriched and -depleted layers. IHP can also be produced by soil microbiota (Liu et al., 2018) and in calcareous soils, it can be decomposed by soil microbiota (Doolette et al., 2010). However, the IHP in our soils was not related to the levels of microbial biomass or soluble C and it was not an important variable for the discrimination between C-enriched and C-depleted soils in the first PCA axis, perhaps because of the IHP content in the compost itself. Indeed, higher abundances of IHP have been found during the composting process (Hashimoto et al., 2014) and after adding manures to soils (Li et al., 2022a).
Orthophosphate diesters are mainly composed of phospholipids and nucleic acids, mostly derived from microorganisms (Vincent et al., 2013). After compost addition, we found slight increases in orthophosphate diesters only in the organic C-enriched surface soils. Increases in orthophosphate diesters have also been observed after the application of crop residues, with the highest increase associated with the lowest application rates (Wu et al., 2021); this finding was related to higher structural stability in soils after crop residue application and microbial activity. In our soils, more abundant orthophosphate diesters were correlated with higher levels of organic C (r = 0.579, p = 0.019) but not microbial or soluble C.
The soil content of other-monoesters, which are readily mineralizable (Xin et al., 2019), may depend on the presence of microbial cells (Bünemann et al., 2008). In the present study, the other-monoesters slightly increased in compost-amended soils in both tested layers and both organic C-enriched and -depleted soils, irrespective of the amount of microbial biomass C. Other-monoesters have been found in compost (Hashimoto et al., 2014) and may be related to microbial populations in compost-amended soils. However, in our experiment, the other-monoesters did not correlate with microbial biomass C or soluble C, but they increased in incubated crop-free surface soils, even though the microbial biomass C was half that of the initial soil.
Effects of growing crops in compost-amended soils
Growing crops for two years did not change the total organic C or soluble C in the soil, but the C in the microbial biomass was higher. P extracted by the crops was about 608 mg pot− 1, which is of the same order of magnitude as the reduction in total P measured in soils from pots growing crops, considering a bulk density of 1.5 g cm− 3. Crop growing did not significantly change the total Po. The concentration of total P decreased by about 57 mg P kg− 1 in both layers, indicating that P uptake relative to soil mass was similar in both organic C-enriched and -depleted layers. Nevertheless, the crops obtained three-quarters of their P uptake from the bottom organic C-depleted layer, which had a bigger volume. This suggests that the plants were able to use P from subsurface organic C-depleted soils despite its lower availability. However, according to local reference tables, the availability of Pi in our uncropped compost-amended soils was high in both layers (Villar and Villar, 2016). Growing crops for two years reduced Pi availability (NaHCO3-Pi) by a similar magnitude in both layers, irrespective of the organic C content. While Pi levels in the organic C-rich top layer remained high after crop growth (53.21 mg Pi kg− 1), they were reduced to the medium-low range in the bottom layer (16.12 mg Pi kg− 1). As in other studies (Shen et al., 2014b; Zhan et al., 2015), we found higher levels of NaHCO3-Pi in organic C-rich versus organic C-poor soils. However, the difference in available P did not change the magnitude of the reduction in NaHCO3-Pi after crop culture.
Growing crops decreased the levels of both NaHCO3-Pi and NaHCO3-Po. Their reduction has been reported in other studies after crop growth, but only in low-available P soils, with increases found in soils with high P availability (Romanyà et al., 2017; Barrow et al., 2022) or with a positive P balance (Zhan et al., 2015). In the latter study, the higher P availability and P reserves after adding manures did not change this trend and retained Pi appeared to be the main source of P for plants in all cases. In agreement with this, our results show a reduction in labile NaHCO3-Pi after growing crops, even in the organically fertilised subsurface soils with low levels of P (9.37 mg NaHCO3-Pi kg− 1). Such decreases coincided with a reduction in total Pi and EDTA-NaOH-Pi, suggesting that retained Pi pools may be a source of P for plants. Large differences in total soluble C and microbial C between layers did not affect the decrease in Pi that occurred in both studied layers. It would therefore seem that the crop use of available Pi pools is mostly regulated by plant demand rather than by the pools of P and organic C in the soil.
Crop growing did not induce any changes in total Po pools in any of the studied soils and the total Po/Pi ratio increased, suggesting higher retention of Po in both organic C-enriched soils and -depleted soils. Po forms have been reported as dominant in low-P soils (Recena et al., 2015). The mobilization of Po in crop-growing soils may thus be confined to less retained Po forms, such as those extracted by EDTA-NaOH and NaHCO3, and may be related to microbial P recycling. Other studies have reported a higher mobilization of Po due to crop-microorganism interactions (Hallama et al., 2021). Po mobilized by soil microbiota of crop-growing soils has been described as the fraction of the EDTA-NaOH-Po that can be mineralized by enzymes (Hallama et al., 2022), which may be increased by cover crops. Although we did not find any alterations in EDTA-NaOH-Po in crop-growing soils, the absence of changes in the EDTA-NaOH Po/Pi ratio together with a significant reduction of EDTA-NaOH-Pi could indicate that a fraction of EDTA-NaOH-Po may also contribute to supplying the crop with P.
The large decrease in NaHCO3-Po and the NaHCO3 Po/Pi ratio in organic C-enriched soil suggests that this form of Po can be used by plants. It has already been proposed that NaHCO3-Po contributes to plant nutrition in soils with relatively low levels of NaHCO3-Pi (Romanyà and Rovira, 2007). While the total Po/Pi ratio increased in the presence of crops but without any changes in total Po, significant reductions in both parameters were observed in unamended cropless incubated soils. This suggests that soil microbiota can mineralize Po compounds in the absence of plants. However, this Po depletion coincided with a significant reduction in soil microbial biomass C and the total C/total Po ratio, suggesting a C limitation for the microbiota in unamended cropless soils. This contrasts with the increase in the total Po/Pi ratio, soluble C, and microbial biomass C in compost-amended crop-growing soils compared to cropless soils. It is therefore, possible that higher levels of available C in crop-growing compost-amended soils may enhance microbial processes, contributing to building up Po pools in both organic C-enriched and -depleted soils. It has been previously proposed that the demand for C can drive microbial mineralization of Po in organic C-poor soils (Wang et al., 2016). In the present study, while the total C/total Po ratio was not altered in the crop-growing C-depleted soils, it increased considerably in the organic C-enriched layer, which had the highest total C/total Po ratio. This could indicate that in organic C-enriched soils with high P availability, the availability of organic C does not limit Po-mineralizing microbiota and that C inputs from crop rooting systems may contribute to building up soil organic C. This is in agreement with the results of a previous study obtained after legumes were grown in field soils with high P availability (Romanyà et al., 2017).
Increases in other-monoester compounds in organic C-enriched cropped soils amended with exogenous organic C have been related to microbial biomass and processes (Li et al., 2022a). In the present study, levels of other-monoesters increased in the organic C-enriched layer of crop-growing soil and incubated plant-free soils. In the latter, this coincided with a reduced total C/total Po ratio, and microbial and soluble C, whereas the opposite trend was found in soils growing crops. Root exudates have been reported to enhance soil microbiota in the rhizosphere (Keiluweit et al., 2015), where the mucilage is very rich in carbohydrates (Knee et al., 2001; Fernández et al., 2017). Other studies in intensively managed agricultural soils have found higher levels of P and microbial biomass C after adding glucose, mainly in organic C-enriched soils (Xu et al., 2020). This may be an indication that microbial biomass in the cropped organic C-enriched layer is more likely to accumulate readily mineralizable P compounds than the microbiota in the organic C-depleted layer in our study.
Although IHP can be produced by plants (Richardson et al., 2000), and in the PCA analysis IHP strongly discriminated between cropping and non-cropping organic C-enriched soils, no significant increases in IHP were observed in the crop-growing soils. As IHP is known to be an important source of P for plants (Steffens et al., 2010), its increase may have been counteracted by plant usage, mainly in organic C-enriched soils. Despite their relationship with microbial processes (Condron et al., 2005; Turner and Newman, 2005), orthophosphate diesters were found to be only marginally affected by crops. Similarly, pyrophosphate was detected in cropping soils both enriched and depleted in organic C. Other authors have related higher levels of pyrophosphate to increases in microbial biomass (Condron et al., 1985; Bedrock et al., 1994; Wei et al., 2018), mainly associated with P compounds from saprophytic and mycorrhizal fungi (Makarov et al., 2005; Bunemann et al., 2008).