A long-term field experiment was conducted over a twenty year period to examine the effects of three different soil management regimes (Abandonment, Fallow and Cropping) and eight nutrient management regimes under Cropping on soil organic carbon (SOC), N and P levels, microbial biomass, and enzymatic activities related to C, N, and P cycling in a loess soil. The nutrient management regimes examined involved treatment with various combinations of inorganic nitrogen, phosphorus, and potassium fertilizers (N, NP, PK, and NPK), as well as combinations of NPK fertilizers with either residual plant material (SNPK) or manure (MN1PK and MN2PK). Abandonment resulted in greater levels of soil microbial biomass than did Cropping but similar levels of enzyme activity were observed under both regimes. The Fallow regime gave significantly lower soil organic carbon levels and enzyme activities than did Cropping. Within the Cropping system, the treatments containning nitrogen and phophorus significantly improved SOC, N and P levels and also increased microbial biomass and enzyme activity relative to the control. In general, the highest values of the tested soil parameters were observed under the M2NPK treatment. With the exception of invertase, the activity of all soil enzymes tested correlated significantly with SOC and microbial biomass. It was concluded that the use of fertilization regimes involving applying organic material in conjunction with NPK fertilizers should be encouraged in order to maintain or improve the chemical and biological properties of the tested loess soil and to thereby increase its productivity.

Black soils in the Northeast Plain of China are characterized by high organic carbon (C) density and storage, which can influence C sequestration in local cropland ecosystems. Using GIS, this study analyzed the temporal and spatial distribution and variation of cropland organic C density and storage in the arable layer (0-20 cm) of the major black soil regions in China's Northeast Plain (specifically across the study region of Hailun, Shuangcheng and Gongzhuling counties) over the past 30 years. The results indicate an overall downward trend in soil organic C (SOC) density (SOCd, mean decrease of 0.64 kg m -2 ) and storage (SOCs, decrease of 4.65 Tg )* 1 for the major black soil regions tested during the study period. The mean decrease in SOCd for Hailun, Shuangcheng and Gongzhuling counties was 0.68 kg m -2 , 0.18 kg m -2 and 1.05 kg m -2 , respectively, with total SOC decreases of 2.30 Tg, 0.49 Tg and 1.86 Tg, respectively. SOCd decreased to a greater extent during the first twenty years in Hailun and Shuangcheng relative to Gongzhuling. Moreover, SOCd tended to stabilize and then slightly increase during the last ten years. There was a downward trend in SOCd and SOCs over the past 30 years for almost all soil types, but the most serious decline in SOCs was observed in black soils, which account for 57.92% of the cropland soil area in all study regions and which lost up to 2.91 Tg of SOC. Therefore, the major black soil region in Northeast China was presenting a C resource trend over the past 20-30 years.