Effects of Pinus caribaea forests on the C, N, P, and S status of Brazilian savanna Oxisols

Abstract

Abstract The accumulation of humus in soils under Pinus caribaea Morelet forests may increase the availability of organically bound N, P, and S to plants and thus improve soil fertility. To test this hypothesis, we compared C, N, P, and S concentrations and forms in soil solid phase and soil solution under Pinus (PI) and natural savanna vegetation ( Cerrado , CE). We sampled the 0–0.15, 0.15–0.3, 0.3–0.8, 0.8–1.2, and 1.2–2 m soil depth layers of Oxisols from three spatially separated plots under CE and PI; at each plot, soil solution at 0.15, 0.3, 0.8, 1.2, and 2 m soil depth and under PI additionally below the organic layer was collected during two rainy seasons (21 October–28 April 1997/1998 and 1998/1999). Under PI, the organic layer stored more C (24.5 t ha −1 ), N (2.6 t ha −1 ), P (140 kg ha −1 ), and S (261 kg ha −1 ) than under CE (1.2 t C ha −1 , 23 kg N ha −1 , 1.2 kg P ha −1 , 2.3 kg S ha −1 ). No significant difference in C, N, P, and S concentrations between CE and PI in the surface horizons of the mineral soil (C: 20.7–23.4 g kg −1 , N: 1.2–1.5 g kg −1 , P: 0.16–0.26 g kg −1 , S: 0.18–0.21 g kg −1 ) was observed. However, at 1.2–2 m soil depth C and organic N (N org ) concentrations were significantly higher under PI (C: 6.9 g kg −1 , N org : 0.54 g kg −1 ) than under CE (C: 6.4 g kg −1 , N org : 0.41 g kg −1 ). C/P and C/S ratios in soil solid phase decreased with increasing soil depth under CE and PI, those of C/N org only under PI, leading to narrower C/N org ratios under PI (12.9) than under CE (15.6) at 1.2–2 m soil depth. The result indicated enhanced N transport to the subsoil under PI. Total organic C concentrations (TOC) in the soil solution were similar under CE and PI and throughout the mineral soil. Under PI, TOC concentrations decreased sharply between the litter leachate (30 mg C l −1 ) and 0.15 m soil depth (5 mg C l −1 ) indicating strong TOC sorption onto the surface mineral soil. Total N concentrations in the soil solution were 11–23 times higher under PI than under CE. Organic N concentrations also decreased between the litter leachate and 0.15 m soil depth under PI but NO 3 − concentrations increased from the litter leachate to 1.2 m soil depth and decreased between 1.2 and 2 soil depth, probably due to enhanced anion sorption at 2 m soil depth. No S and P was detectable in any solution sample. Higher C, N, and S storages and higher N availability under PI indicated that PI forests improve C, N, P, and S supply for plants in subsequent cultures and may therefore be considered sustainable with respect to organically bound nutrients.