Soil organic carbon sequestration and fertility response to newly-built terraces with organic manure and mineral fertilizer in a semi-arid environment

Terraces are a popular and widely-distributed engineered agricultural landscape in mountainous regions. In this study, the effects of adding manure on soil organic C (SOC) sequestration, soil aggregate distribution, C/N ratio and activities of enzyme were evaluated in newly-built terraces over 7 growing seasons from 2004 to 2010 on the semi-arid Loess Plateau of China. Experimental treatments including CK (control treatment with no fertilizer), NP (nitrogen and phosphorus), M (manure) and MNP (manure, nitrogen and phosphorus) were employed with a field pea spring wheat potato cropping system. SOC built up quickly in M and MNP treatments, while applying fertilizer alone (NP) was not effective in increasing soil C. The C sequestration rates are high in the early stages in manure treatments, and then decrease with consequent increases of soil C pool and finally became zero as soil C reaches an equilibrium level. The simulation results showed that the maximum increase of SOC in newly-built terraces was 3.4 g kg(-1). Organic amendment significantly increased soil water stable macro-aggregates (>0.25 mm). The largest aggregates (> 2 mm) in manure treatments have important implications for C sequestration, with 29.4% and 30.6% of total SOC in M and MNP, respectively. Application of fertilizer alone could not form the largest aggregates (> 2 mm) in the 7 years. SOC content was significantly higher in M and MNP than in CK and NP in all soil aggregate size classes. Manure treatments improved soil C/N ratio, while fertilizer treatment (NP) decreased soil C/N ratio. A significant sigmoid relationship was found between C/N ratio and cumulative inputs of SOC and TN in manure treatments, and a significant logistic relationship was found C/N ratio and cumulative TN inputs in NP treatment. After 7 years, fertilizer alone was as important as manure for improving activity of urease. Manure treatments improved activities of alkaline phosphatase and D-glucosidase compared with no manure treatments. The overall results show that applying fertilizer alone (NP) was not effective in accelerating the build-up of fertility and adding manure is essential in improving soil fertility and accelerating the build-up of fertility in newly-built terraces in a semi-arid environment.