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Soil labile organic carbon fractions and soil organic carbon stocks as affected by long-term organic and mineral fertilization regimes in the North ChinaPlain.

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A series of soil N mineralization indices were evaluated using 153 samples chosen from arable fields representing a wide range of soil types, management practices, and climatic zones. These indices were compared against potentially mineralizable N (N0) determined by aerobic incubation at 25°C for 24 wk. Three different pools of mineralizable N were recognized: Pool I, the mineralization flush on rewetting in the first 2 wk; Pool II, gross N mineralization in the next 22 wk; and Pool III, the potentially mineralizable N, predicted from the fitted curve, that did not mineralize during the incubation period. Pool I was highly correlated with CaCl2–N, KCl‐NH4, and KCl‐NO3, which extract soil mineral N. Pool III was significantly correlated with ultraviolet absorbance of NaHCO3 extract at 205 and 260 nm (NaHCO3–205 and −260), Illinois soil N test, NaOH direct‐distillation N, and hot KCl‐NH4, which mostly extract hydrolyzable organic N. All indices except the mineral N based methods, phosphate‐borate buffer method, and microbial biomass C were significantly related to N0, which includes both Pools II and III. The NaHCO3–260, NaOH direct‐distillation N, and Illinois soil N test had the highest correlations with N0 (r2 = 0.74, 0.61, and 0. 51, respectively). Total organic C and N represent long‐term changes in N0 and were almost as effective in predicting N0 as the other indices (r2 = 0.60 and 0.67, respectively); however, they would be expected to be less sensitive to short‐term changes in N0 due to changes in soil management practices and history.

Reduced tillage is increasingly promoted to improve sustainability and productivity of agricultural systems. Nonetheless, adoption of reduced tillage by organic farmers has been slow due to concerns about nutrient supply, soil structure, and weeds that may limit yields. Here, we compiled the results from both published and unpublished research comparing deep or shallow inversion tillage, with various categories of reduced tillage under organic management. Shallow refers to less than 25 cm. We found that (1) division