Spatial variability of soil fertility under agroforestry system and native forest in eastern Amazonia, Brazil

The usage of spatial tools might be helpful in the optimization of decision-making regarding soil management, with technologies that assist in the interpretation of information related to soil fertility. Therefore, the present study evaluated the spatial variability of chemical attributes of the soil under an agroforestry system compared to a native forest in the municipality of Tomé-açu, Eastern Amazon, Brazil. Soil samples were performed at 36 points arranged in a 55 x 55 m grid. The soils were prepared and submitted to analysis in order to determine pH in H2O, exchangeable calcium, magnesium, potassium and aluminium, available phosphorus, potential acidity, organic matter, bases saturation and aluminium saturation. For each soil attribute, the spherical, gaussian and exponential models were adjusted. After the semivariograms fitting, data interpolation for assessment of spatial variability of the variables was performed through ordinary kriging. The spherical and gaussian models were the most efficient models in estimation of soil attributes spatial variability, in most cases. Most of variables presented a regular spatial variability in their respective kriging maps, with some exceptions. In general, the kriging maps can be used, and we can take them as logistical maps for management and intervention practices in order to improve the soil fertility in the study areas. The results principal components indicate the need for integrated management of soil chemical attributes, with localized application of acidity correctors, fertilizers and other types of incomes, using the spatial variability of these fertility variables.

Efeitos de diferentes usos da terra sobre as características químicas de um latossolo amarelo do estado do Pará / Effect of land use and management on chemical properties of a yellow latosol in Pará State, Brazil

Estudou-se o efeito de diferentes tipos de cobertura vegetal e manejo sobre algumas características químicas de um Latossolo Amarelo, textura média, da Estação Experimental "José Haroldo", da CEPLAC, Marituba-PA. Foram amostradas parcelas sob floresta secundária (FS) explorada seletivamente, de aproximadamente 40 anos; de derruba e queima (DQ) da mesma floresta secundária; sob cultura de cacau (CC); e, sob área de pastagem abandonada (PA). Em cada área estudada foram abertos três perfis de 2 m de profundidade, coletando-se amostras de solo dos horizontes Ap, AB, BA e B21, totalizando 48 amostras compostas, que foram analisadas para pH, carbono orgânico, nitrogênio total (N), fósforo disponível (P) e bases trocáveis (K+, Ca2+, Mg2+ e Na+). Foram utilizadas análises de regressão para verificar as correlações existentes entre pH, nitrogênio total (N), fósforo disponível (P), matéria orgânica (MO) e soma de bases (SB). Houve aumento no pH, na soma de bases e no fósforo disponível e diminuição no conteúdo de matéria orgânica em área de derruba e queima; aumento no conteúdo de matéria orgânica em área de pastagem abandonada; aumento do conteúdo de P do solo em área de cultivo de cacau. Não houve efeito do tipo de cobertura vegetal no N-total do solo. Houve correlações positivas entre o pH x matéria orgânica do solo e desta com os conteúdos de N e P do solo.

The study was conducted at the Experimental Station "José Haroldo" of CEPLAC, Benevides, PA, to determine changes in chemical properties of a Yellow Latosol, medium texture, under different land cover systems. Samples were collected from the Ap, AB, BA, B21 horizons of the soil under a 40-year old secondary forest (FS), recent slash and burned secondary forest (DQ), cocoa cropping (CC) and degraded grassland (PA). Soil samples were analyzed for pH, organic carbon (SOC), total nitrogen (N), available phosphorus (P) and exchangeable bases (K+, Ca2+, Mg2+, Na+). Relationships among the soil chemical properties were determined by regression analysis. Results showed that available P, SOC and total N decreased with soil depth in all management systems. Highest values of available P, SOC and total N were found in the Ap horizon following the sequence cocoa > grassland > secondary forest. There was significant correlation between pH and SOC, and between SOC and soil N, and available P.