ObjectiveThis study is to study the relationship between content and distribution of aggregate associated C and N and aggregate stability, and further analyze the difference of aggregate disruption mechanism between farmland and forested land, expectantly provide theoretical foundation for improving soil productivity by returning farmland to forestland and other vegetation rehabilitation measures in black soil region.
MethodIn our study, a sloping farmland (preceding crop is corn) and an adjacent sloping forestland (artificial Pinus sylvestris forest) in typical black soil region of northeastern China were selected and each slope was divided into the upper slope position, upper to middle slope position, middle slope position, middle to lower slope position, and lower slope position along the three sampling lines which parallelled to the slope aspect. The soil samples were sieved individually and separated into five aggregate-size fractions (2-5 mm, 1-2 mm, 0.5-1 mm, 0.25-0.5 mm, < 0.25 mm) using the dry sieving and wet sieving method, then the percentage of aggregate disruption (PAD) of each fraction was calculated. Each fraction of soil water stable aggregates was passed through 0.25 mm sieve to determine aggregate-associated organic carbon, total nitrogen, ammonium-N, nitrate-N, and the ratio of ammonium-N to nitrate-N (ANR).
ResultIn sloping forestland, aggregate-associated C and N deposited in the lower slope position and accumulated in smaller aggregates; aggregate-associated C and C/N in farmland was significantly lower comparing with which in forestland; but aggregate-associated N did not show a distinct trend with the change of slope position and aggregate size. Aggregate-associated ammonium-N predominated in < 1 mm aggregates and which of forestland was higher than farmland; however, aggregate-associated nitrate-N mainly concentrated in >1 mm aggregates. PAD was affected obviously by land use type, aggregate size and slope position: PAD of farmland was significantly lower than forestland especially for >1 mm; PAD of 2-5 mm in upper, middle, middle to lower slope positions was obviously higher than other aggregate sizes in the same slope position.Aggregate-associated C and N did not present a significantly negative and positive relationship with PAD at significance level of 0.05; but C/N, ANR and ammonium-N were negatively correlated with PAD, and both ANR and C/N were in a lower level when PAD was greater than 40%.
ConclusionDistribution of aggregate-associated C and N and aggregate stability between farmland and forestland are caused by the difference of nutrient restitution and soil erosion environment. Aggregate stability of black soil is determined by the interaction of aggregate-associated C and N, not the individual concentration of aggregate-associated C or N. The integrated indicators C/N and ANR, which are closely related to soil microorganism, have significant influence on soil aggregate disruption; moreover, ammonium-N used by microorganism preferentially in the soil environment also shows a significant correlation with aggregate stability. In the future, we should pay more attention to the relationship between microorganism activity and soil aggregate stability.
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