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United States Department of Agriculture

Agricultural Research Service

Title: Short-Term Effects Fo Tillage on Mineralization of N and C in Soil under Organic Farming

Authors
item Kristensen, H - DANISH INST. OF AG. SCI.
item Debosz, K - DANISH INST. OF AG. SCI.
item McCarty, Gregory

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 14, 2003
Publication Date: N/A

Interpretive Summary: The stability of soil organic matter is important in terms of the ability of agriculture take up carbon dioxide from the atmosphere and reduce global warming. Tillage has been associated with decreasing soil organic matter and consequently a deterioration of soil quality. Studies of agricultural systems under long-term management involving plow-tillage and no-tillage practices as well as undisturbed native soils have demonstrated that tillage has marked influence on soil organic matter content with release of inorganic nitrogen and carbon. The nature of soil nitrogen protected by the intact structure of soil and released by tillage was studied in the laboratory by use of N isotope labeling of soil N followed by simulation of tillage by sieving. The simulation of tillage by breakup of the larger soil structure caused a small release of N from a pool which was otherwise protected against microbial decomposition. The use of soil crushing for disruption of larger macroaggregates and chemical fumigation for perturbation of the microbial biomass increased the release from both active and protected N pools. But the composition of nitrogen pools released by these treatments were indistinguishable, suggesting that the released N originated from the same pool, namely the soil microbial biomass pool. This study provides further evidence of the importance of soil microorganisms in the stability of soil organic matter within agricultural systems.

Technical Abstract: Tillage is known to decrease soil organic N and C pools with negative consequences for soil quality. This decrease is thought partly to be caused by exposure of protected organic matter to microbial degradation by the disturbance of soil structure. Little is known, however, about the short- term effects of tillage on release of N and C from soils, and the release needs to be related to microbial activity of soil organic pools. The dilution of 15-N label in the inorganic N pool mineralized during 14 days incubation of the sieved samples relative to intact samples was used for calculation of active and protected N release. The simulation of tillage caused a small release of N from a pool which was otherwise protected against microbial degradation. The use of soil crushing for disruption of larger macroaggregates (>425 micron) and chloroform fumigation for perturbation of the microbial biomass increased the release from both active and protected N pools. The relative contribution from the protected N pool was, however, similar in the three treatments (22-27%), thus the pools subjected to mineralization were characterized by similar degrees of protection. When comparing the isotopic composition, the pools of N mineralized were indistinguishable, suggesting that the released N originated from the same pool, that is the soil microbial biomass. The results also indicate that the microbial biomass contained a significant pool of protected (inactive) N, and that the larger macroaggregates were equally important for protection of soil N from microbial degradation as the whole-soil. The macroaggregates nevertheless held a large pool of labile N (active + protected) which may potentially be released by soil structure disturbance.

Last Modified: 12/21/2014
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