Site and landscape position-dependent effects of vegetation removal on soil nitrogen mineralization across five sites on China?s Loess Plateau

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.