How Soil Organic Carbon Fractions Affect N 2 O Emissions in a Long-Term Integrated Crop-Livestock System: A Case Study

The nitrous oxide (N2O) emissions in agricultural systems are influenced by edaphoclimatic conditions, and the availability of soil organic matter (SOM) is a key factor in this process. Understand the relationship between SOM fractions and N2O emissions in cultivated soils is fundamental to the sustainable management of tropical soils. However, this relationship remains unclear. The objective of this study was to evaluate the accumulation of labile and stable fractions of SOM and their relations with N2O emissions in a 24-year field study that represents farm conditions in the Cerrado region. The following hypotheses were considered: (i) conservation systems protect SOM, avoiding its rapid decomposition and, consequently, reducing losses of N2O to the atmosphere; (ii) conservation systems favor the increase of labile and stable fractions of SOM, which has the effect of reducing the N2O emission in the soil. The following land-use systems were assessed: no-tillage with integrated crop-livestock system (NT1); no-tillage with continuous cropping (NT2); and conventional system (CT). An area of native vegetation of Cerrado was used as a reference. Nitrous oxide emissions were quantified over a period of 509 days, covering two agricultural years with soybean crop followed by sorghum and corn as a second crop in 2014/2015 and 2015/2016 agricultural years, respectively. Soil carbon fractions (labile and stable) and carbon in different classes of soil aggregates were also determined. The cumulative N2O emissions were larger in CT, intermediate in NT systems, and smaller in the Cerrado area. Among the agricultural systems, lower cumulative N2O emissions were observed in NT1, because of the greatest buildup of carbon in its most stable fractions and occluded in aggregates. From PCA results, it is possible to conclude that aggregation is a key factor that correlates with N2O emissions from soil. Thus, NT1 showed the largest average diameter of aggregates and presented the lowest N2O emissions among agroecosystems. Although the conservation systems show a greater microbial population, stable fractions of carbon are predominant, which decreases availability for the soil microbiota, which justifies lower rates of SOM mineralization and, consequently, the lowest N2O emissions.