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Highlights • Converting grassland to shrubland increased soil N availability and mineralization. • The positive effect on soil mineral N was higher in lower slope than upper slope. • The positive effect on soil mineral N was similar across soil textures. • The effect on net N mineralization varied with slope position and soil texture. Abstract Grassland afforestation, which is a major form of land-use change, has expanded considerably in the last few decades. However, the effect of conversion from grassland to shrubland on soil nitrogen (N) cycling and availability with regard to soil textures and slope positions is unclear. This lack of clarity has hindered our understanding of how grassland afforestation influences soil N availability and transformation on a larger spatial scale. In this study, we compared the net N mineralization rates and mineral N concentrations in soils from native grassland and under legume shrub (Caragana korshinskii Kom.) with contrasting soil textures (sandy-loam vs. loamy-sand) and slope positions (upper vs. lower) in the northern region of China's Loess Plateau. The objective of this study is to understand how soil texture and slope position regulate the response of soil N cycling to grassland afforestation. The results showed that the concentrations of soil nitrate and total mineral N and the rates of net nitrification and mineralization were higher in soils under shrub (0.64 ± 0.10 g N m−2, 1.01 ± 0.11 g N m−2, 23.19 ± 5.22 mg N m−2 d-1 and 22.07 ± 5.44 mg N m−2 d-1) than in grassland (0.22 ± 0.03 g N m−2, 0.56 ± 0.04 g N m−2, 1.48 ± 1.51 mg N m−2 d-1 and 1.36 ± 1.96 mg N m−2 d-1). However, soil ammonium concentration and net ammonification rate were not affected by grassland afforestation. The effect of afforestation on nitrate and total mineral N concentrations was greater in the lower slope than in the upper slope but was similar between the sandy-loam and loamy-sand soils. The effect of this land-use change on net nitrification and mineralization rates varied with soil texture and slope position, with greater effects in the upper slope than in the lower slope in more clayey soils, but with the opposite influencing pattern in more sandy soils. These results suggested that planting C. korshinskii in grassland ecosystems increased soil N mineralization and availability. More importantly, soil texture and slope position should be considered when assessing the effects of grassland afforestation in complex landscape conditions. [ABSTRACT FROM AUTHOR]

Abstract Shrubs play a vital role in ecosystem restoration in the desert transition zone, which suffers intensive desertification, land degradation, and nitrogen (N) limitation. To evaluate the effects of different shrub species on soil N availability, transformation, and their seasonal dynamics, we determined mineral N contents and mineralization rates in soils under three common shrub species (Artemisia ordosica Krasch., Caragana korshinskii Kom., and Salix psammophila C.) and in bare land during the growing season in the desert-loess transition zone in northwestern China. The average contents of soil nitrate N and mineral N were 153% and 55% greater, respectively, and cumulative net N nitrification and mineralization were 101.2 and 121.0 mg N m−2 higher, respectively, in soils beneath C. korshinskii than in bare land (P < 0.05) at the 0–10 cm depth over the growing season. The net N mineralization rate in 0–10 cm depth was significantly higher in soils under C. korshinskii than that in bare land during 21 Jun-23 Jul and 23 Aug-15 Oct of the growing season. However, C. korshinskii did not influence soil N metrics at the 10–20 cm depth. The incorporation of A. ordosica and S. psammophila into bare land did not influence soil N availability or transformation rates. Our results demonstrated that C. korshinskii was more preferable than A. ordosica and S. psammophila in ameliorating soil N cycling in the desert-loess transition zone. Highlights • Xeric shrub species: Artemisia ordosica , Caragana korshinskii , Salix psammophila • The fragile transition zone between the desert and the Loess Plateau of China • C. korshinskii increased mineral N content and net N mineralization rate • Effects of C. korshinskii were greater in dry and cold period and the 0–10 cm soil. • A. ordosica and S. psammophila did not influence soil N mineralization. [ABSTRACT FROM AUTHOR]

Abstract The presence of vegetation has significant effects on soil nitrogen (N) turnover. However, whether these effects vary with slope aspect, vegetation type and time of season were not understood in previous studies, precluding our ability to understand how vegetation affects soil N cycling in sloping land. In this study, we investigated the effects of vegetation presence (plots with vs. without vegetation), vegetation type (grassland vs. woodland) and slope aspect (east vs. west slope) on soil N turnover during the growing season in sloping lands of the Loess Plateau, China. Soil metrics measured included concentrations of ammonium (NH 4 +), nitrate (NO 3 −) and total mineral N (Min-N) in soil solution and rates of ammonification (R a), nitrification (R n) and net N mineralization (R m). We hypothesized that the presence of vegetation would deplete the soil Min-N pool and decrease R m and that these effects would be greater in east slopes and woodlands than in west slopes and grasslands. In partial support of these hypotheses, vegetation presence decreased soil Min-N concentration but did not affect R m. NH 4 + and NO 3 − contributed similarly to Min-N, while R n dominated R m. The effects of vegetation presence on soil mineral N and N mineralization varied with the time of the season but were not related to the vegetation type and slope aspect. The soil Min-N and R m were significantly higher in woodlands, east slopes and 0–10 cm depth than grasslands, west slopes and 10–20 cm depth, respectively. The R a and NH 4 + increased, while the R n , R m , NO 3 − and Min-N decreased with increasing soil moisture. These results indicated that soil Min-N and R m in the sloping lands of the Loess Plateau consistently respond to vegetation presence across slope aspect and vegetation type, and were regulated by soil moisture. Highlights • Vegetation decreased soil inorganic N pool because of plant uptake. • Effects of vegetation presence (plots with vs. without vegetation) on soil mineral N and N mineralization varied with season. • Effects of vegetation presence on N turnover were irrespective to vegetation type and slope aspect. • Soil mineral N pool and mineralization rate were higher in woodland and east than grassland and west slope. [ABSTRACT FROM AUTHOR]

• Soil in eroding zone had higher mineral N and net mineralization rates than in depositional zone. • Vegetation removal increased soil mineral N but not affected soil net N mineralization rates. • The effects of vegetation removal on mineral N pools depended on landscape position, site, and season. • The responses of net N mineralization rates to vegetation removal were consistent across landscape positions and sites, but varied with season. The soil nitrogen (N) availability is a key limiting element of terrestrial ecosystem productivity and functioning, especially in fragile ecosystems (e.g., eroding environments). However, to date, little is known about how vegetation and erosion individually or jointly affect soil N mineralization under field conditions across a range of sites varying in soil and climate properties on a long-term observational scale. In this study, we elucidated how soil net N mineralization responds to vegetation removal (plots with vs. without vegetation) and landscape position (eroding vs. depositional zones) during a two-year manipulation experiment using an in situ incubation method on the China's Loess Plateau. The results indicated that the nitrate and total mineral N concentrations and net nitrification and mineralization rates were higher in the eroding zone (2.74 ± 0.12 mg kg−1, 4.47 ± 0.13 mg kg−1, 0.046 ± 0.005 mg kg−1 d-1 and 0.046 ± 0.006 mg kg−1 d-1) than those in the depositional zone (2.01 ± 0.10 mg kg−1, 3.80 ± 0.12 mg kg−1, 0.030 ± 0.005 mg kg−1 d-1 and 0.032 ± 0.006 mg kg−1 d-1) on the China's Loess Plateau. Vegetation removal resulted in an increase in the nitrate (2.88 ± 0.13 vs. 1.87 ± 0.09 mg kg−1) and total mineral N concentrations (4.56 ± 0.14 vs. 3.71 ± 0.11 mg kg−1), but did not affect the soil net N mineralization rates on the China's Loess Plateau. The effects of vegetation removal on soil mineral N pools depended on the site, season and landscape position, with stronger effects in the eroding zone than those in the depositional zone. However, the responses of the net N mineralization rates to vegetation removal were consistent across the various landscape positions and sites, but varied with the season. Variance partitioning revealed that soil properties rather than climate and vegetation conditions were the most dominant contributors to the variations of soil N mineralization in both the eroding and depositional zones, explaining 37.6–64.6% of the variation. These results emphasized the substantial spatial (landscape position and site) and temporal (seasonal) dependence of the effects of vegetation removal on soil N mineralization at large scale. [ABSTRACT FROM AUTHOR]

Soil erosion can dramatically alter soil nitrogen (N) cycling, but it has rarely been examined under in-situ conditions due to the complicated interaction between erosion and vegetation. Comparing the effects of vegetation presence at eroding and depositional sites would provide a direct understanding of such effects. Herein, we investigated how soil N dynamics (mineral N pools and net N mineralization rates) responded to vegetation presence (plot with vs. without vegetation), landscape position (eroding vs. depositional sites) and vegetation type (grassland vs. woodland) during a two-year manipulation experiment using an in-situ incubation method in the hilly-gully region of China's Loess Plateau. The results showed that soils on eroding site had higher concentrations of nitrate (3.820 ± 0.302 vs. 2.642 ± 0.280 mg kg-1) and mineral N (5.853 ± 0.320 vs. 4.308 ± 0.287 mg kg-1) but similar rates of net nitrification (0.057 ± 0.013 vs. 0.051 ± 0.011 mg kg-1 d-1) and mineralization (0.056 ± 0.014 vs. 0.056 ± 0.013 mg kg-1 d-1) in comparison to depositional site in all grassland plots. The concentrations of soil nitrate and mineral N and rates of net nitrification and mineralization in legume woodland were 26%, 19%, 88% and 64% higher than that in grassland in all slope plots, respectively. Although vegetation presence significantly decreased soil nitrate (−57%) and total mineral N (−27%) concentrations, it did not affect the rates of net nitrification and mineralization. The effects of vegetation presence on soil net N mineralization occurred irrespective of landscape position and vegetation type(P > 0.05), but varied with season or year, with greater effects in 2017 growing season than in the non-growing season and 2018 growing season. Redundancy analysis and hierarchical partitioning indicated that the above- and blow-ground biomass accounted for most of the variation in soil N mineralization. These results suggested that the variations in soil mineral N and net N mineralization in relation to vegetation presence in this eroding watershed were consistent across different landscape positions and vegetation types, but showed intra- and inter-annual variability. • Vegetation presence decreased soil mineral N pools because of plant uptake. • Soil mineral N concentrations in eroding site were higher than deposition site for the grassland. • Soil mineral N and mineralization rates were higher in woodland than grassland for the eroding sites. • Effects of vegetation presence on soil mineral N and mineralization rates were irrespective to landscape position and vegetation type, but varied with season or year. [ABSTRACT FROM AUTHOR]

Re-vegetation of cropland is an important measure for carbon (C) sequestration, while the combined effects of land-use change, re-vegetation type and time on soil organic C (OC) turnover and temperature sensitivity have been less studied. Here, we investigated soil cumulative OC mineralization (C min), specific OC mineralization (C min /C 0 , normalized by initial OC) and temperature sensitivity (Q 10) with an 84-d incubation experiment after the conversion of cropland to artificial grassland (6, 10, 15 and 20 yr), to natural grassland (6, 10 and 20 yr) and to pea shrub woodland (10, 20 and 40 yr) in the agro-pastoral ecotone of northern China. Results showed that re-vegetation significantly increased C min (from 38.21 to 57.79 mg kg−1 soil, +51%) at the 0–10 cm depth but not the 10–20 cm depth, with greater increases after the conversion of cropland to artificial grassland (+77%) than the other two patterns but similar among different conversion years for each pattern. Variations in C min among different re-vegetation patterns and times were mostly explained by changes in total nitrogen concentration (TN) in the macro-aggregate fraction. Across all treatments, 6.20–20.29 g kg−1 OC was mineralized during the incubation and Q 10 was 1.16–2.40. The C min /C 0 and Q 10 changed not significantly during the post-agricultural re-vegetation chronosequences for each pattern, which were mostly due to the unchanged TN concentration in silt and clay fraction. Re-vegetation also changed the dependence of C min , C min /C 0 on N availability, with significant relationships in re-vegetated artificial grassland and pea shrub woodland which may due to the influence of legumes on microbes. In conclusion, OC mineralization increased with conversion, varying by re-vegetation type, while OC decomposability and temperature sensitivity were stable regardless of re-vegetation type and time, indicating the similar biodegradability and stability in face of warming. • OC mineralization and Q 10 were tested in varied re-vegetation types and times. • Cropland re-vegetation increased C min especially for artificial grassland. • •N in macro-aggregate explained about 50% of cumulative OC mineralization. • •OC biodegradability and Q 10 were similar during the post agricultural restoration. [ABSTRACT FROM AUTHOR]

• Afforestation increased significantly soil OC and N mineralization. • The increase of OC and N mineralization were larger in sites with high clay content. • The temperature sensitivity decreased in sites with relative high clay content. • Soil texture regulated OC and N mineralization and their temperature sensitivity. Afforestation on abandoned cropland has great potential to increase soil organic carbon (OC) and nitrogen (N). Identifying the effect of afforestation on mineralization of OC and N as well as their temperature sensitivity at large spatial and temporal scales is essential to understand soil OC and N dynamics. In this study, we measured mineralization of soil OC and N at standard temperatures (15 and 25 °C) by laboratory incubation, and calculated their temperature sensitivity (Q 10). Soil samples were collected from cropland (0 years, selected as a control) and adjacent woodlands with different afforestation age (10, 20 and 30 years) at five sites from north to south on China's Loess Plateau. The objectives were to understand how long-term afforestation affects OC and N mineralization and their Q 10 and whether such effects depend on afforestation age and soil texture. The results showed that afforestation increased OC and N mineralization, but this effect was not affected by afforestation age (10–30 years). The Q 10 of OC and N mineralization increased in the first 10 years of afforestation but decreased in the late stage. Furthermore, the increases of OC and N mineralization by afforestation were higher at sites with fine-textured soils than those at sites with coarse-textured soils. However, the Q 10 increased at sites with a relatively high sand content but decreased or remained unchanged at sites with a relatively high clay content. The changes in OC and N mineralization and their Q 10 were related to soil OC and N contents, pH and clay content. Overall, we concluded that long-term afforestation would contribute to the stability of soil OC and N in afforested ecosystems. Soil texture is an important factor controlling OC and N mineralization and their Q 10 , and should be considered when predicting soil OC and N dynamics in response to temperature change in different landscapes. [ABSTRACT FROM AUTHOR]

• Simulated erosion decreased mineralization of OC and N in soils • Decrease in OC and N mineralization was mainly occurred in small aggregate sizes • Changes in OC and N availability determined decrease in OC and N mineralization • The OC and N mineralization increased with OC and N contents but texture independent Soil erosion has important influence on soil processes and particularly soil nutrient dynamics, which in turn affects the terrestrial C and N cycling and greenhouse gases emission. However, the responses of carbon (C) and nitrogen (N) cycling in soils and soil fractions to erosion remain largely unclear, hindering the precise prediction of the fate of C and N in the terrestrial ecosystem. Herein, we compared the mineralization of organic carbon (OC) and N at two different temperatures (15 and 25 °C) in bulk soils and aggregate fractions of 19 soils before and after a simulated rainstorm. The temperature sensitivity (Q 10) of OC and N mineralization was calculated. Soil samples were collected from 9 sites across China, with soil types of Alfisols, Inceptisols, Mollisols, Ultisols, and land uses of croplands, grasslands, and woodlands. The soil aggregate fractions were >2, 2-1, 1-0.25 and <0.25 mm, respectively. We hypothesized that erosion will decrease OC and N mineralization due to the loss of labile organic matter from soils and that the effects of erosion would decrease with clay content and vary with different aggregate fractions. In support of these hypotheses, the cumulative mineralized OC and N (C m and N m) in the soils and aggregate fractions were decreased of 78-85% and 37-52% after erosion, and the temperature sensitivities of OC and N mineralization were decreased of 80-84% and 4-43%, respectively. The effects of erosion varied with site where the soil was collected. The decreases in C m and N m mainly occurred in small aggregate sizes (1-0.25 and <0.25 mm fractions). The C m and N m in bulk soils and aggregates increased significantly with the contents of OC, N, labile OC and mineral N but were texture independent. These results highlighted significantly reduction of OC and N mineralization at eroding sites, which should be carefully considered when assessing emissions of greenhouse gases in eroding environments. [ABSTRACT FROM AUTHOR]