Submitted to: Soil and Tillage Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/27/2006
Publication Date: 1/1/2007
Citation: Green, V.S., Stott, D.E., Cruz, J.C., Curi, N. 2007. Tillage impacts on soil biological activity and aggregate stability in Brazilian Cerrado Oxisols. International Journal of Soil and Tillage Research. 92:114-121.
Interpretive Summary: Mechanized agriculture is increasing rapidly in the Cerrado region of Brazil. This region covers about two million sq. kilometers (0.8 million sq. miles) in central Brazil. Thirty years ago, the area served primarily as rangeland with a few cows per sq. mile. Recent advances in soil fertilization and management has allowed up to 70% of this area to be classified as suitable for agriculture, but so far, only about 10% is currently used as cropland, and 17% as planted grasslands. Increasing production in this area alleviates pressures on more sensitive ecosystems such as the Amazon rainforests to the north and the Pantanal wetlands to the southwest. However, increasing production in this area is not free of ecological impact. Many Amazon River tributaries flow through this area, and increased production could lead to increased sedimentation of the river ecosystems. Soils form the basis of most terrestrial ecosystems. The influence of soils is felt even in the oceans in the form of nutrients and sediments transported from the land by streams and rivers. With that in mind, ARS and Brazilian scientists undertook an experiment to determine what the impact of two decades of agricultural production has had on a soil typical of the Cerrado region. The soil was a red Latosol, classified as a Rhodic Hapludox (Oxisol) in the US taxonomic system, and was typical of the most common soils of the Cerrado that cover 46% of the area. We looked at three tillage regimes, including a disk plow system, which would be similar to the conventional chisel/disk system in the US Corn Belt, a disk harrow system that is equivalent to a minimum tillage system, and a no-till system. The study area had been tilled for 20 years, and in the respective tillage management systems for the last five years. We found that soil biological activity as measured through soil enzyme activities and carbon and nitrogen mineralization, to be greater in no-till that the other managed systems. This trend is similar to the one found in the more-studied temperate soils. Soil enzymatic activity was highly correlated with nitrogen and only weakly related to soil organic carbon. This differs from temperate soils, which generally show a high correlation between enzyme activity and soil carbon. This is probably due to these soils being limited in nitrogen and phosphorus rather than readily available carbon, as is typical of temperate soils. Iron and aluminum are considered, at least in the English language scientific literature, to be the dominating soil aggregating agent in Oxisols. Our results showed that soil organic matter also played an important role in the stability of aggregates in tropical soils, with tillage and subsequent loss in organic matter resulting in decreases in soil organic matter. This work shows that to remain productive and reduce environmental impacts, the soils of the Cerrado should be managed with minimum or no-tillage systems.
Technical Abstract: Mechanized agriculture is increasing rapidly in the Cerrado region of Brazil, increasing concerns about water quality, off-site impacts, and sustainability. Our objective was to determine the impact of tillage on soil biological activity and aggregate stability on an Oxisol typical of the region. Three different tillage practices and an area under native vegetation were examined. Five different soil enzyme activities, C- and N-mineralization, organic C, total N, and aggregate stability were determined. Total N, acid phosphatase, arylamidase, and C- and N-mineralization were the most sensitive to changes in tillage management. For each of these assays the no-till system had greater concentrations or activities than disk plow. No-till also had greater levels of total N, and C- and N-mineralization when compared to the conservation till system (disk harrow). Total N ranged from 1.77 to 2.18 mg cm-3. C- and N-mineralization ranged from 2.77 to 6.83 and 0.035 to 0.100 mg cm-3 and 2.77 to 6.83 mg cm-3, respectively. Enzyme activity in all sites was weakly correlated with organic C and strongly correlated with total N. Aggregate stability was related to soil organic carbon increases, contrary to literature available on Oxisols. The results from this research show the importance of reducing tillage as a means of increasing soil biological activity of the topsoil. By understanding the effects of tillage on soil biological properties of tropical soils, management systems can be implemented that improve the natural nutrient cycling processes and improve soil structure, resulting in increased agricultural sustainability in tropical ecosystems.