Freeze-thaw cycles changes soil nitrogen in a Mollisol sloping field in Northeast China

Freeze-thaw-cycles (FTCs) change the dynamics of soil physiochemical and biological processes, and could change the soil nitrogen (N) heterogeneity in sloping fields. In this study, both continuous cropping of corn (CC) and corn-soybean rotation (CS), with-straw-amendment (SA) and with-no-straw-amendment, were considered to investigate how FTCs change soil total N (TN) and available N (alkali-hydrolyzable N, AN) at different slope positions in the Mollisol region of Northeast China. When a large amount of snow fell, after FTCs: (1) TN decreased by 40-79% at all slope positions under CC and CS, mainly influenced by snowmelt-erosion, nitrification-denitrification and N-upward-movement, and (2) AN decreased at both the top slope (51-64%) and back slope (36-81%) positions, mainly influenced by snowmelt-erosion, nitrification-denitrification and N-upward-movement, but increased (2-65%) under CS at the bottom slope position, mainly due to deposition, N-upward-movement, nitrification-denitrification and previous crop. In contrast, when less snow fell, after FTCs: (1) N transformation changed both TN and AN on the slopes, mainly influenced by microorganisms, N-upward-movement, and previous crops, and (2) AN decreased by 20-26% at the bottom slope position under both CC and CS, mainly influenced by nitrification-denitrification and N-upward-movement, but increased by 111-116% at the back slope position under CS. SA did not change the TN distribution on the Mollisol farmland slope after FTCs in a short time period, but it helped sustain a high AN content (4.5-8.6%) in the soil. TN loss was usually severe after FTCs, and should be controlled urgently to increase soil fertility and reduce environmental pollution.