Mudanças da matéria orgânica de neossolos quartzarênicos em agroecossistemas orgânicos e convencional

Abstract

Soil organic matter (SOM) is an important reservoir of nutrients and carbon (C) and plays animportant role in the structure and functions of the ecosystem. However, this role can bemodified by different land use systems, as conventional and organic use. The objective was toevaluate, quantitative and qualitatively, different physical fractions of SOM: particulate organicmatter (POM), associated with minerals (MAOM) and dissolved organic matter (DOM) inorganic agroecosystems (Organic Banana Culture - BNo and Organic Horticulture - HTo) andconventional (Conventional Agriculture - AGcv) cultivated on Nessolos Quartzarênicos(Entisols (Quartzipsamments)). An area of natural vegetation (VN) was evaluated withreference. Compared to VN, HTo, BNo, and AGcv reduced C stocks by 12.4%; 12.5%, and 7.1%,and C-MOAM by 13.3%, 10.2%, and 11.8%, respectively. HTo showed higher stocks of totalC and N and of MAOM, POM, and DOM at 0-15 cm, however, there is a high C saturationdeficit in the subsurface soil (35-100 cm). Total N stocks were lower in AGcv. Total C and Nstocks from MAOM, found in the subsurface soil (>15 cm to 100 cm) of VN were decisive forthe higher stocks at 0-100 cm compared to agricultural systems. Noteworthy in BNo, a lowerN-POM stock than the others (0-100 cm). The chemical composition of DOM was altered bythe management systems in terms of differences in UV-Vis absorbance. The land use systemsshowed a reduction of aromaticity (FTIR) with increasing soil depth in all SOM fractions,except for DOM, where only VN showed an increase of aromatic compounds at depth. Thethermochemolysis analysis showed the relative abundance of organic compounds and presentedtheir percentage differences between use systems, with suberin predominating in MAOM,especially in VN in the subsoil (>15 cm to 100 cm). HTo, at the surface (0-15 cm) is formed byan organic matter with more diversified compounds in both POM and MAOM, while in BNoincreased much more the proportion of n-alkanes in POM with depth much more. Theabundance of phospholipid fatty acids (PLFAs) did not show significant differences betweenuse systems, but between depths did, as it occurred for fungi and bacteria, especially in theorganic management systems. BNo and AGcv showed a lower abundance of fungi (F) at 0-15cm. The proportion of fungi/bacteria was higher in AGcv (55-100 cm), while G+/G- was higherin HTo and BNo (0-15 cm), and from 15-55 cm in BNo only. As for the physical properties, itwas observed that BNo showed the lowest percentage of stable macroaggregates, especiallywith VN. Organic vegetable growing systems did not increase or maintain SOM in the subsoil(>15 cm to 100 cm) in Entisols (Quartzipsamments), even with continuous use of organiccomposts, crop rotation, and green manure. However, organic management improved the physical, chemical, and biological attributes of the topsoil (0-15 cm). Future evaluations ofplants with more aggressive root systems will be important increase the C and N in the subsoil(>15 cm to 100 cm) for organic farming systems.