Long-term land use influenced stoichiometric patterns of C, N, P and microbial biomass along soil depth in Mollisol at Northeast China

Changes in soil carbon (SOC), nitrogen (TN), phosphorus (TP) and microbial biomass play a vital in plant growth, and their stoichiometric ratios have great significance on soil nutrient balance and ecosystem functions. In this sense, it is urgent to advance our understanding of how long-term land use affects SOC, TN, TP, microbial biomass and their stoichiometric ratios along the deep soil profile. This study aims to investigate the effects of land use on soil C, N and P stoichiometry, and their relationships with microbial biomass in mollisol at Northeast China. The long-term land use included woodland (poplar forest as control, PF), maize cropping (MC) and greenhouse vegetable farming (GVF).

The results indicated that soil SOC and TN concentrations in PF were markedly higher than those in MC, and the vertical distribution of SOC and TN concentration showed an inverted triangle trend as the soil deepens. Soil C/N fluctuated in narrower ranges along the soil layers (0–5, 5–10, 10-20, 20–40, 40–60, 60-80, 80-100cm). Both soil C/P and N/P showed significant variability in different land use types, and soil N/P decreased with increased depth of soil layers. Compared to woodland, maze cropping decreased the C content, but increased P content, resulting in the decreases of C:N, C:P and N:P ratios. Hence, maize cropping soil was relatively limited by C and N but enriched with P. By contrast, the C, N and P contents in greenhouse vegetable farming soils were all increased compared to woodland soils, but larger increases of N and P contents resulted into the decreases in C:N and C:P ratios. The intensive fertilization was probably the main contributor to the higher P content, and consequently lower C:N:P ratios in MC and GVF soils. Both the microbial biomass C (MBC) and N (MBN) showed a decreasing trend with the increase of soil depth, and all soil layers from high to low was: GVF > MC > PF. Our results revealed that, the more intensive agricultural practices and stronger biological and geochemical processes lead to more pronounced differences of soil C, N, and P contents and their stoichiometric ratios among land-use types.