Abstract
The implementation of conservationist systems of cultivation, with less soil turning and a greater deposition of plant residue, has increased the stocks of C and N in soil and humic substances, but may reduce the quality of the organic matter (SOM). The aim of this study was to evaluate the storage of carbon and nitrogen in plant residue, humic substances and soil at three depths (0-0.2, 0.2-0.4 and 0.4-0.6 m) and under five systems of cultivation: an old banana plantation (14 years cultivation), renovated banana plantation (renewed 1.5 years earlier), sugarcane plantation (two years cultivation), pasture (three-year-old Colonial grass), crotalaria juncea (cultivated yearly for five years) and native forest, as a reference of a natural semi-arid environment. The dry weight of the plant residue deposited on the soil was determined, together with the levels and stocks of total carbon (C) and total nitrogen (TN), the stocks of C and N in the humic substances (fulvic acid, humic acid and humin fractions), and the C/N and C-HF/(C-FAF+C-FAH) ratios. The greatest levels of C were seen in the plant residue from the renewed banana plantation, pasture and crotalaria, but the deposition of dry matter and the stocks of C and TN were higher in the plant residue of the native forest. In the soil, the largest stocks of C and TN were found in the surface layers (0-0.2 and 0.2-0.4 m). The stock of C and TN, and of C in the humic fractions of the SOM did not differ between the majority of the cropping systems or the native forest, indicating the maintenance of C and N in the cultivated soils compared to the native vegetation. Cropping systems that include banana, sugarcane and crotalaria increase the C stock in the humin fraction and the degree of humification of the SOM at most of the soil depths under evaluation.
Highlights
Agricultural crops located in semi-arid regions of Brazil have presented difficulties in maintaining or increasing stocks of soil organic carbon
The residue from the crotalaria had the highest levels of N (2.5 dag kg-1) among the cropping systems (Table 2), due to the legume presenting symbiosis with N-fixing microorganisms, which are able to increase N in the plant tissue
The highest stock of total nitrogen (TN) was seen in the crotalaria, with 0.34 t ha-1 N, differing statistically from the other cropping systems, but lower than the stock of N obtained in the native forest residue (Table 2)
Summary
Agricultural crops located in semi-arid regions of Brazil have presented difficulties in maintaining or increasing stocks of soil organic carbon. Barbera et al (2012) reported the lowest, or no, difference in the storage of C and TN in the soil (0-15 cm layer) after 19 years of direct planting compared to a conventional system, in a semiarid region of Italy. Those authors attributed this result to the lower susceptibility of the SOM to degradation, due to the recalcitrance of the plant residue (wheat) and the presence of soils with a high clay content (47.1 dag kg-1), which affords the SOM greater physical protection
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