Improving soil aggregation, aggregate‐associated C and N, and enzyme activities by green manure crops in the Loess Plateau of China

Abstract Green manure and fertilization practices have substantial impacts on soil chemical and biological properties over time. A field experiment was conducted on Cumulic Haplustoll to investigate the effects of leguminous green manure (LGM) crops and nitrogen (N) fertilizer on dry‐stable soil aggregate distributions and stability, the concentrations of organic carbon (OC) and N in the bulk soil and separated aggregate fractions, and soil enzyme activities in dry croplands on the Loess Plateau of China. Species tested as LGM crops were Huai bean ( Glycine ussuriensis Regel et Maack .), soybean ( G. max ( L. ) Merr .) and mung bean ( Phaseolus radiatus L.). Two rates of N fertilizer (0 and 162 kg N ha −1 ) applied to winter wheat sown after LGM was incorporated into surface soil. Results showed that the inclusion of LGM crops for 6 years significantly increased the total nitrogen (TN), soil organic carbon (SOC), aggregate‐associated N and OC concentrations, enhanced the values of aggregate content with >0.25 mm size (R 0.25 ) and mean weight diameter (MWD), and boosted soil enzyme (invertase, urease, dehydrogenase and aryl‐sulfatase) activities at the 0–20‐cm depth compared to the bare fallow system under dryland conditions. The long‐term application of N fertilizer substantially increased the proportion of macroaggregates (>5.0 mm), MWD and N contents in the 0.25–5‐mm size aggregates, and promoted the relative activities of invertase, alkaline phosphatase, dehydrogenase and aryl‐sulphatase, but decreased the urease activity. The correlations among macroaggregate fractions and soil chemical properties, aggregate stability and enzyme activities were significant and positive; however, the correlations between small aggregate fractions (<2 mm) and corresponding parameters were negative, or there were no correlations. Overall, combining green manuring practices with suitable N fertilizer application should be recommended to improve the soil quality in dryland regions similar to the Loess Plateau of China. Highlights Green manure crops significantly increased soil enzyme activities, SOC and total N contents. Green manure crops increased macroaggregate (>5 mm) formation and stability. Application of N fertilizer significantly increased the proportions and stability of macroaggregates. Green manure crops along with N fertilizer helped to build better soil quality in a dry loess cropland.