Characteristics Of In-situ Mineralization Of Soil Nitrogen Under Erosion Environment In The Loess Plateau

Soil erosion is the extensive and influential form of soil degradation around the world,resulting in the redistribution of nutrients such as soil organic carbon and topsoil,reshaping the surface environment,affecting the productivity of terrestrial ecosystem and soil biogeochemical cycling?such as soil nitrogen cycling?.Additionally,soil nitrogen?N?mineralization is one of the fundamental processes of soil N cycling,which not only controls N availability and supply to plants,but also affects ecosystem productivity and regulates global carbon balance.The Loess Plateau has fragmented terrain and widespread sloping land where suffers severe soil erosion,which provides a favorable platform for studying soil N cycling in erosion environment.Different small watersheds of the Loess Plateau,namely Shenmu Liudaogou watershed,Suide xindiangou watershed,Ansai Zhifanggou watershed,Guyuan Huochagou watershed and Changwu Wangdonggou watershed were selected to carry out in situ N mineralization experiments in eroding and depositional sites of every small watershed.The temporal and spatial differentiation characteristics of soil N mineralization in the erosion environment and their response to seasonality,environmental factors and vegetation status were clarified,and the variations in these responses between eroding and depositional sites were further elucidated.The main conclusions were as follows:?1?Soil organic carbon?SOC?,total nitrogen?TN?,pH,bulk density?BD?,clay content,moisture and temperature were significantly discrepant across all five sites?five watersheds:Liudaogou?LD?,Xindiangou?XD?,Zhifanggou?ZF?,Huochawan?HC?and Wangdonggou?WD??.Generally,the contents of SOC and TN in the eroding site were significantly higher than that in the depositional site,and related to site.The soil pH,clay content and BD in the depositional site were significantly higher than those in the eroding site and regardless of site.Soil moisture in the eroding site was significantly higher,while soil temperature was significantly lower than in the depositional site.?2?The soil inorganic nitrogen pool is dominated by ammonium?NH₄⁺?,but existing site-heterogeneity.The LD is dominated by ammonium,but ammonium and nitrate?NO₃^-?collectively composed of inorganic N pool in XD and HC sites,and were jointly dominated in the eroding site and 0-10 cm soil depth in ZF site.The eroding site in WD site is dominated by ammonium,while ammonium and nitrate collectively predominated the inorganic N pool in the depositional site.In addition,nitrification dominated soil nitrogen mineralization across five sites.?3?No significant difference was observed in the soil inorganic N pool between the eroding and depositional site,but was related to site and sampling date?p<0.05?,and was independent of soil depth?p>0.05?.For example,there is no significant difference in LD and HC sites.The content of NH₄⁺in the eroding site of XD site is not significantly different from that in the depositional site,while the contents of NO₃^-and inorganic nitrogen?Min-N?is significantly higher than that in the depositional site.The NH₄⁺content in eroding site was significantly lower,while the NO₃^-content was significantly higher than that in the depositional site.There was no significant difference in inorganic nitrogen?Min-N?content.The NH₄⁺content in the eroding site of WD site was significantly higher,whereas the NO₃^-content was significantly lower than that in the depositional site,but there was no significant difference in inorganic N?Min-N?content between eroding and depositional sites.?4?Vegetation presence significantly depleted soil inorganic N pool,but was related to site and sampling date?p<0.05?.The contents of NO₃^-and Min-N in plots without vegetation in SM site was significantly higher,but the NH₄⁺content was significantly lower than that in the plots with vegetation.The NH₄⁺content was significantly higher,but no variation in contents of NO₃^-and Min-N in plots with vegetation in XD site than that in plots without vegetation.There was no significant difference in NH₄⁺content;the contents of NO₃^-and Min-N in the plots without vegetation in ZF,HC and WD sites was significantly higher than that in plots with vegetation.?5?Topography and vegetation presence did not affect net rates of ammonization?Ra?,nitrification?Rn?and mineralization?Rm?,but was related to site and incubation period?p<0.05?,and was independent of soil depth?p>0.05?.For example,no significant difference of Ra,Rn and Rm were observed between the eroding and depositional sites in LD site,but Rn and Rm in plots with vegetation were significantly higher than that in plots without vegetation;Rn and Rm in the XD site were not affected by the vegetation presence,but Rn and Rm in eroding site were significantly higher than that in depositional site;Ra,Rn and Rm in the depositional site of ZF site were significantly higher than that in the eroding site,and vegetation presence significantly weakened the Rn,but not affected the Ra and Rm.In addition,the Ra,Rn and Rm were not significantly affected by topography and vegetation in HC site,whereas all of them in eroding site in WD site were significantly higher than those in depositional site,but not affected by vegetation presence.?6?The contents of NH₄⁺,NO₃^-and Min-N in soil were positively correlated with SOC and TN?p<0.01?.Soil NH₄⁺content was positively correlated with carbon to nitrogen ratio?C:N?,while NO₃^-and Min-N content were significantly negatively correlated with C:N?p<0.05?.There was a significant negative correlation between soil NH₄⁺,Min-N and pH value?p<0.01?,but no statistical correlation between soil bulk density and clay content?p>0.05?.The Rn was affected by interaction of site and soil moisture,clay content,and was affected by interaction of topography and soil moisture?p<0.05?.The effects of topography and site on Rm were related to soil moisture?p<0.05?.It is indicated that the difference in soil nitrogen mineralization across sites and between eroding and depositional site is regulated by soil moisture and clay content.