Synchronous soil N and C sequestration
Previous studies have attributed land-use change as one of the most important human effects on the rate of changes in TON and TOC concentrations and stocks (Tan et al., 2007; Li et al., 2012; Guan et al., 2015). The current study showed significant gains in soil TOC and TON after afforestation on previously cultivated FL (Fig. 1 and Fig. 2). Lemenih et al. (2005) and Zhang et al. (2020) showed 19% and 15%, respectively, increases in soil TON stocks after afforestation on agricultural land. Similarly, Laganière et al. (2010) and Assefa et al. (2017) advocated higher soil TON stocks in tree plantations is because trees have plenty of litterfall and extensive root system, which increase organic matter inputs and reduce losses from soil erosion. Similarly, 2.243-fold increase in leaf litter stock and 16.917-fold increase in fine root biomass was observed in H28, as compared to H8. Similar to other studies (Chang et al., 2014; Assefa et al., 2017; Liu et al., 2018), the current study also showed that the gains in TOC and TON stocks contents strongly depend on the number of post afforestation years.
Initially, losses of soil TON stocks were observed in H8 (20–80 cm) than in the original FL (Fig. 2). Wang et al. (2019) and Chang et al. (2014) also reported a decline in soil TON and TOC contents during the early stages of FL restoration because of the disturbance during site preparation, the lower productivity of new vegetation, the scarcity of leaf litter and low root biomass. Li et al. (2012) reported significant soil C and N stock increases were found 30–50 years after afforestation, and before these time points, C and N stocks were either depleted or unchanged. Vesterdal et al. (2002) attributed it to young stands' low organic matter inputs and high rates of C decomposition inherited from farmland to meet the needs of rapidly growing stands. According to Compton and Boone (2000), there is typically a time lag for soil C accumulation because, during afforestation in North Carolina, nearly all of the increase in ecosystem C went into standing biomass, but not soil C. Furthermore, according to Xia et al. (2021) and Darby et al. (2020), as plant biomass accumulation slows in mature stands, soils may re-accumulate N that is no longer required for new plant biomass, supporting larger soil N storage in older stands. Similarly, in the current study, 28-years later, afforestation resulted in cumulative (0–100 cm) 23.128 Mg ha–1 gains in TON stocks at an accumulation rate of 0.121 Mg ha–1 yr–1, which is 1.167-fold higher than H8 (Fig. 2), implying Z. bungeanum plantations are capable of restoring soil N and C in this arid region.
Soil TON follows similar temporal patterns of SOC sequestration, and slight improvements in TON stocks can significantly increase soil SOC gains (Leblans et al., 2014; Liu et al., 2018). Bingham and Cotrufo (2016) further established that soil N plays an indispensable role in C sequestration by improving the stability of SOM, particularly in the mineral soil layer. Similarly, the current study showed that TOC sequestration was 6.796-fold higher than TON sequestration in H28 (Fig. 2). Zhang et al. (2020) showed SOC sequestration is synchronous to TON. In the present study, this is also evident from the significantly (P < 0.05) positive correlation observed between TOC and TON (r = 0.948) contents and further strengthened by the RDA (Fig. 7 and Fig. 8).
Similar to several other studies (e.g., Chen et al., 2010; Justine et al., 2017; Xia et al., 2021), results of the current study also revealed that higher TON and TOC concentrations were found in the upper 0–40 cm soil layers compared to deep soil horizons (Fig. 1 and Fig. 2). However, the gains in soil C and N concentrations below 0–40 cm were potentially higher compared to the topsoil. Gregory et al. (2016) also reported higher C and N sequestration potential of deep soil. Liao et al. (2020) reported that C and N pools in the subsoil (below 0–30 cm) are assumed to be more stable than the topsoil due to differences in the composition of SOM and environmental conditions. In addition, Li et al. (2019) reported that soil N is one of the determinative factors of restoring degraded ecosystems. Leblans et al. (2014) also advocated that N accumulation accelerates soil development. The current study revealed strong interaction in soil C and N under Z. bungeanum plantations which improved significantly as the age of the plantations increased. Thus, planting Z. bungeanum is an effective approach for restoring ecosystems N gains and C sequestration under the current land-use change scenario.
Post land-use changes in concentrations of soil organic N fractions and TIN availability
Soil organic N fractions constitute biologically active N pools, act as sources of easily mineralized N, and contribute to the stable N pools (Batlle-Aguilar et al., 2011). These organic N pools are susceptible to land-use change (Magid et al., 2010; Zhou et al., 2020). In the current study, land-use change from FL to Z. bungeanum plantations and GL evidently favored gains in MBN, AHN, SON, PON, MAON, LFON, and HFON contents (Fig. 3 and Fig. 4). Mao and Zeng (2010) investigated the impact of cropland conversion to tree-based plantations and reported improved soil quality and fertility was due to increased POM-N and MBN contents. Similarly, in the present study, afforestation with Z. bungeanum enhanced resulted in overall (0–100 cm) gains of 1.619-folds and 1.591-folds in PON and MBN contents respectively, in H28 compared to the original FL (Fig. 3 and Fig. 4). Soil OM is both a substrate for and a product of soil microorganisms resultantly, microbial necromass is estimated to be the most abundant form of SOM (Kallenbach et al., 2016). Wild et al. (2015) and Li et al. (2019) further argued MBN affects N dynamics because the microbial demand for C and N drives SOM decomposition and ecosystem C and N sequestration. Similarly, in the current study, MBN was the most abundant labile organic N fraction following afforestation, suggesting that FL conversion to Z. bungeanum plantations is conducive to soil N restoration.
Several studies have also regarded MBN, AHN, SON, and PON as potential sources of mineralized N i.e., plant-available TIN in soil solutions, which in the current study is also supported by the RDA (Fig. 7) and correlation analysis (Fig. 8). Similar to other studies (e.g., Mendham et al., 2004; Belay-Tedla et al., 2009; Mao and Zeng, 2010; Maharjan et al., 2017; Li et al., 2019), soil TIN contents were most significantly correlated to AHN, PON, MBN, and LFON (Fig. 8). Increased soil labile organic N fractions accumulation upon afforestation and FL abandonment is linked with the reduced disturbance caused by tillage and increased biomass return to the ecosystem in the form of above and belowground plant litter (Magid et al., 2010; Asseefa et al., 2017; Zhou et al., 2020). Increments in biomass and litter return thus ultimately labile organic N fractions accumulation is directly controlled by the post-afforestation duration (Mendham et al., 2004). In the present study, concentrations of labile N fractions increased significantly (P < 0.05) as the plantations aged (Fig. 3), which coincides with the other similar studies (Mao and Zeng, 2010; Darby et al., 2020; Xia et al., 2021), and indicated positive impacts of Z. bungeanum plantations soil N pool stability and N supply capacity.
The scientific community is divided on the role of LFON and PON in the course of N cycling. Several reports support these two fractions play an important role in stabilizing soil N pools as well as serve as short term N sink (Compton and Boone, 2002; Mendham et al., 2004; Tan et al., 2007) and upon demand serve as highly labile N source (Gregorich et al., 2006; Yan et al., 2007). In the current study, LFON (r = 0.981) and PON (r = 0.983) correlated positively with N contents suggesting their key role in the N sequestration (Fig. 8). The increased PON and LFON post-afforestation can positively affect soil physical structure and aggregation and make soil organic C and N pools more stable (Mendham et al., 2004; Tan et al., 2007; Gartzia-Bengoetxea et al., 2009). Soil LFON and PON cumulative contents (0–100 cm) increased by 1.329-fold and 1.394-fold in H28 compared to the youngest H8 plantations, respectively (Fig. 4). Liao et al. (2020), and Gregory et al. (2016) attributed increased stability of soil N to the increased PON and LFON fractions in the subsurface soils. Similarly, results of the current study showed that soil depth significantly affected vertical dynamics of labile organic N fractions, and with the plantations age significant (P < 0.05) gains in HFON, LFON, PON, SON, AHN and MBN contents were observed across all soil depths (Fig. 3 and Fig. 4).
Land-use change improved labile soil organic N concentrations across all soil depths; however, increases in HFON and MAON concentrations were much greater and improved continuously with the age of the Z. bungeanum plantations (Fig. 4). The fractions of N consisting of SOM intimately associated with minerals, decrease microbial and enzymatic capacity to decompose organic substrates (Plaza et al., 2019), thus serve as sinks of organic N in the soil (Whalen et al., 2000; Tan et al., 2007;), and in principal are the long-term N sources for plants and microbes (Qiu et al., 2015). Hassink (1997) and Cai et al. (2021) showed increased soil MAON contents significantly improved N pool stability and capacity of soils to preserve N and C long-term. In the present study this is supported by RDA (Fig. 7), and confirmed by the significant (P < 0.05) positive correlation observed for HFON (r = 0.997, r = 0.933) and MAON (r = 0.999 and r = 0.943) with soil TON and TOC, respectively (Fig. 8). Thus, it can be asserted that post-afforestation increases in HFON and MAON played a significant role in increasing the stability and storage capacity of TON sinks in soils. The land-use-related variations in these pools are generally linked to the changes in OM inputs with post-afforestation years, as increased aboveground biomass output and litter inputs increase with the plantation age (Laganière et al., 2010; Qiu et al., 2015; Feyissa et al., 2020; Xia et al., 2021). Cai et al. (2021) reported increased MAON contents improved soil aggregation and structural stability; previous studies have also shown Z. bungeanum plantations as an effective measure to cease soil erosion and reclaim degraded soils (Cheng et al., 2015).
Additionally, while the topsoil contained higher concentrations of HFON and MAON, the increases in the subsurface layers were much more significant than those in the topsoil (Fig. 4), indicating that Z. bungeanum plantings are well-suited for ecosystem restoration projects. However, to date, there has not been a comprehensive examination of the temporal and vertical dynamics of MAON and HFON following afforestation, particularly under arid conditions. Therefore, variations in MAON and HFON must be included in future studies to gain insights into the N sink stabilization paradigm for long-term N sequestration in intensively managed plantations.
Heterogeneous vertical distribution dynamics of LON, NLON and NPMI
The soil N pool management index (NPMI) is derived from Blair et al. (1995), who first combined the soil TOC pool and C lability index to evaluate the capacity of management systems in promoting soil quality. Later on, Westerhof et al. (1998) and Gong et al. (2011) adapted this for soil N and suggested NPMI is a useful index to evaluate and compare the changes that occur in total and labile N as a result of management practices, with an emphasis on the changes in labile and non-labile N in SOM. Thus NPMI can provide a useful parameter to assess the capacity of management systems to improve soil quality (Westerhof et al., 1998; Gong et al., 2011). However, these indexes have largely been ignored in ecological restoration studies. The current study showed that converting FL to Z. bungeanum plantations and GL considerably improved ecosystems N lability, NPI, LI, and NPMI (Fig. 6). Based on the previous reports (Gami et al., 2009; Lal, 2016; Jiang et al., 2021), the higher value of the C pool management index implies that soil is being restored, improved and maintained. The strong stoichiometric association between soil SOC and TON, also suggests the values of NPMI may also serve as valuable indicator to reflect the status of soil quality.
The trend of NPMI among soil horizons can be further established by the relative soil LON and NLON stocks distribution along the soil depth. Soil LON and NLON exhibited heterogeneous vertical distribution patterns across the soil profile in all land-use systems (Fig. 5). Higher distribution of soil labile N in the topsoil layers significantly increases soil N supply capacity (Chen et al., 2018). In their experiment, Chen et al. (2018) found that labile N in 0–30 cm soil profile correlated positively with N mineralizing enzymes and soil N supply capacity. In the current study, surface soils had higher soil LON compared to the subsurface soil (Fig. 5), similarly, in the present study, the higher value of NPMI for H28 might be due to lack of disturbance and reduced mineralization of labile N pools (Compton and Boone, 2002; Mao and Zeng, 2010; Zhou et al., 2020). Feyissa et al. (2020) suggested that the higher aboveground biomass production and litter inputs could increase substrate inputs and labile C and N concentrations in the topsoil. Previous studies have established that ecosystems, to sequester N and C on a long-term basis, need to stabilize SOM, and one of the mechanisms is by increasing compounds that are biochemically resistant towards microbial decay (Filley et al., 2008; Belay-Tedla et al., 2009; Feyissa et al., 2020). The present study revealed that the NLON was the dominant N form in the soil, and it correlated strongly with the NPMI (r = 0.886, P < 0.05), indicating increased N pool stability and long-term N storage. Previously post-afforestation improvements in soil available nutrients have been regarded as indicators of soil quality and fertility (Gao and Huang, 2020; Tellen and Yerima, 2018). In the current study, NPMI exhibited significant positive correlation with soil available nutrients AP (r = 0.900, P < 0.05), and AK (r = 0.865, P < 0.05). Therefore, these results imply that long-term afforestation with Z. bungeanum plantations on previously cultivated FL could promote soil N sequestration, and NPMI could be used to estimate ecosystem N restoration and the status of soil quality.