Abstract
Soil erosion will cause a degradation in soil nitrogen supplying capacity (SNSC) and manure amendment is an effective way to restored eroded soils. Both labile fractions of soil organic N (SON) and N transformation enzymes are indicators for SNSC, but the effect of manure amendments on labile SON fractions and the relationship between labile SON fractions and enzyme activities remains unclear. In this study, five degrees of erosion were simulated in Mollisols (removal of 0, 5, 10, 20 and 30 cm of topsoil) to analyse the changes in labile SON fractions and nitrogen transformation enzyme activities after 8-year manure amendment. We found that soil total N (TN), labile SON fractions and enzyme activities all increased after manure amendments. The largest labile SON fraction was particle organic nitrogen (POM-N) and the second was light fraction organic nitrogen (LFOM-N), which accounted >60% for TN in total. Correlation analysis showed that both urease and protease activities were significantly correlated with POM-N, LFOM-N, microbial biomass N and dissolvable organic N, indicating that both urease and protease activities can be used to predict labile SON pools and enzyme activities worked similarly in indicating SNSC with labile SON fractions. Altogether, 8-year manure amendment could recover SNSC of lightly eroded Mollisols to natural levels, i.e. erosion depths at 5 cm and 10 cm; however, it is not able to recover SNSC in Mollisols suffering severe erosion.
Highlights
N transformation was regulated by microorganisms and was directly catalysed by the enzymes secreted by microbes[18]
The changes in total N (TN) was similar in both NPK and NPKM treatments, which decreased with the increasing erosion depth
Of the five depths of soil erosion, the highest TN contents were in 0 cm, which were 2.11 g kg−1 in the NPKM treatment and 1.91 g kg−1 in the NPK treatment, whereas the lowest TN contents were in the 30 cm depth, which were 1.40 g kg−1 in the NPKM treatment and 1.29 g kg−1 in the NPK treatment (Fig. 1)
Summary
N transformation was regulated by microorganisms and was directly catalysed by the enzymes secreted by microbes[18]. Proteolysis catalysed by protease is an important process with regard to N-cycling because it is considered to be a rate-limiting step during N mineralization in soils due to the much slower primary phase of protease activities during N mineralization compared with amino acids mineralization[20,22,23]. Both N transformation enzyme activities and labile organic nitrogen fractions can be used as indicators for SNSC, the relationships between them remains unclear. The objectives of this study were to determine i) how labile SON fractions were affected in eroded Mollisols of different erosion depths after maunre amentment, ii) the relationships between nitrogen transformation enzymes activities and labile SON fractions and iii) the restorative effects of manure on Mollisols of different erosion degrees
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