Avaliação da qualidade do solo em sistemas de cultivo irrigado agroecológico e convencional no semiárido do Rio Grande do Norte

Abstract

The Semiarid Northeast Brazil is characterized by hydric deficit, has low soil organic matter, mainly due to erosion, high temperatures and reduced replacement of plant residues and/or organic fertilization. The inadequate use and management in farming systems in the semiarid region may decline the soil's ability to sustain short-time production. This can lead an itinerant agriculture in search of new areas on the caatinga, causing the loss of biodiversity and vulnerability of food production. In this context, currently, there is a demand of the scientific community to develop sustainable agricultural systems and the world population with healthy foods. Therefore, the transition of agricultural systems in Semiarid from conventional to agroecological becomes strategic for resilience, maintaining the soil productivity and less pressure on the caatinga. The agroecological management of soil excludes the use of chemical fertilizers and pesticides, and at the same time, contributed to attachment of organic carbon, nutrient cycling, soil protection and diversity of culture. In this sense, the aim of this study is to prove the relevance of soil quality in irrigated agroecological systems (A area) compared to conventional (C area). For this, it were used areas A and C and other natural vegetation (V area), without human action, for characterization purposes; located in the community of family farmers Sombras Grandes, Caraúbas, RN. The historical use and management of the A, C, V areas were recorded, in order to observe the trend of soil quality indicators from a time frame. In the A and C areas, simple soil samples were used to determine physical (Bd, TP, CDW, FD, AW, AW/TP and Mfc/TP) and chemical (pH, Ca, Mg, K, Na, P assimilable, SB, Al + H, BS, CEC, TOC, CS, EC and ESP) indicators in the layers 0-20 cm and 20-40 cm in the sampling grid of 0.5 ha, in five georeferenced points/year, between 2009 and 2012, totaling 40 samples per area; soil penetration resistance (SPR) with the use of impact penetrometer IAA/Planalsucar - Stolf model, to a depth of 0-20 cm, 0.05 m of intervals, in five points with five replications, between 2009 and 2011; and soil quality index (SQI), integrated quality index (IQI) and soil quality index through soil deterioration (SQId). The analysis of soil macro fauna was performed by Pitfall traps, in order to identify the organisms into functional groups and the wealth index of species of Margalef, Shannon diversity and uniformity of Pielou. It was used the statistical method nonparametric Kruskal Wallis for comparison of soil quality indicators between A and C areas at 5% and 1% significance; the Pearson correlation matrix to test the association between the variables; and principal component analysis (PCA) to check the indicators sensitivity . The use and management in the A area, 0-20 cm layer, it were significantly different and presented better results of soil quality to the C area, in relation to physical (Bd, TP, CDW and FD) and chemical (TOC, CS and Al + H) indicators of soil. The chemical indicators (Ca, Mg, K, CEC, P and SB) didn‟t achieve significant difference between A and C areas, but presented high values able to maintain soil fertility . There were significant difference of PR values in the A area and the best results in relation to the C area, the layers 0-5 cm and 5-10 cm. In A area, layers 0-20 cm and 20-40 cm, there was a significant correlation (p ≤ 0.01) and positive between TOC and CEC (r = 0.71, r= 0.71, respectively). In the A area, 0-20 cm layer, TP showed significant correlation (p ≤ 0.01) with TOC and positive (r = 0.70). In the A area, Factor 1, 0-20 cm layer, it was the main component (MC) that explained most of the variance data corresponding to 34.20%; and soil variables that presented significant correlation (p≤0.01) and positive with the agroecological farming system were Mg (0.716946), SB (0.849990), BS (0.821112), CEC (0.848423), TOC (0.859802), CS (0.825854) and TP (0.776193), indicating that agroecological practices positively affected soil quality indicators over time. The richness, diversity and uniformity of soil fauna indices in A area showed higher values than the C area. The different methodologies and depths studied, soil quality indices were better classified in the A area in relation to C area. The A area had physical indicators, chemical and biological able to maintain and/or increase the soil productive capacity, without the use of chemical fertilizers.