Lower Soil Organic Carbon Stocks Research Articles

Soil carbon stocks under oil palm plantations in Bahia State, Brazil

Oil palm (Elaeis guineensis Jacq.) has been cited as the main raw material for biodiesel production in Brazil. This oilseed is cultivated in the Brazilian Atlantic coast in mixed ‘spontaneous’ agroforest systems and in commercial plantations. Different oil palm cultivations derived from native rain forest can modify the soil organic carbon (C) dynamics. The aim of our study was to evaluate the changes in soil organic carbon (SOC) stocks after conversion of Atlantic forest into oil palm production in Bahia State, Brazil. Soil samplings were carried out in May 2008 and February 2009 in four areas: Native Atlantic Rain Forest (NARF), a mixed secondary forest/agroforestry Spontaneous System (SPSY), oil palm cultivated during 23 (OP23) and 34 years (OP34). The spatial variability of C and nitrogen (N) contents and the differences in soil C stocks between the avenues (inter-rows) and frond piles in oil palm areas were evaluated. We found the highest soil C contents in the region next the oil palm base (1.22% in OP23 and 1.49% in OP34). The soil C stocks were higher in frond piles (1.7 times in OP23 and 2.6 times in OP34) than in the avenues, due to inputs of soil organic matter by pruned fronds. The soil C stocks adjusted for a mass equivalent and different clay content decreased in SPSY. The OP23 area showed lower SOC stocks compared with native system. However, OP34 area showed the highest C stocks among the areas evaluated with an increase of 25% of the amount of SOC storage found under native vegetation. We found SOC storage of 34.7 Mg ha−1 and 66.6 Mg ha−1 under OP23 and OP34, respectively, indicating an increase of soil C stocks in oil palm plantations over time.

Spatial and vertical variation of soil carbon at two grassland sites — Implications for measuring soil carbon stocks

Soil organic carbon (SOC) stocks are a function of the SOC concentration and the bulk density of the fine soil. Both variables are prone to changes and are influenced by abiotic and biotic processes. To determine the effect of variations of both variables on SOC stocks at two grassland sites, one with Stagnic Vertisols and one with Orthoeutric Arenosols, 440 soil cores were sampled down to 60cm depth. Bulk density and C and N concentrations were measured in 5cm intervals (0–10cm depth) and 10cm intervals (10–60cm depth). SOC stocks at the clay rich site with Vertisols were almost twice as high (86t C ha − 1 in 0–60cm depth) as at the sandy site with Arenosols (48t C ha − 1 ). Variations in the SOC stocks were determined by the thickness of the loess layer at the clay rich site. Underlying clay horizons hampered the C translocation into the subsoil which resulted in lower SOC stocks of the whole profile. Semivariograms showed spatial autocorrelations of SOC concentrations within a range of 47 and 131m, respectively. The range of autocorrelation between samples of bulk density was much shorter (39 and 51m). Relative variances in bulk density were 1–2 magnitudes lower than the variability of SOC concentration and decreased rapidly as soil depth increased. The difference in the variation of SOC concentration and bulk density was used to revise the sampling design for SOC stocks. An unequal number of samples, i.e. more SOC concentration samples than bulk density samples, would not necessarily decrease the power of the sampling design to detect SOC stock changes. In contrast, the optimum sampling design for these sites would consist of 33–44% bulk density samples and 56–67% SOC concentration samples.

Changes in soil organic carbon stocks under agriculture in Brazil

Land use conversion from natural to agricultural ecosystems affects concentration and storage of soil organic carbon (SOC) depending on tillage frequency and depth, soil type, climate and other factors. Despite its importance, little is known about the magnitude and consistency of changes in SOC stocks (Mg ha −1) due to management of highly weathered soils in Brazil. From 37 uncultivated/cultivated paired sites in Brazil (most on Oxisols), SOC stocks were calculated for the depths of 0–20 and 0–40 cm. Changes in SOC stocks were calculated for intensive (with annual tillage) and non-intensive (pastures, conservation tillage and perennial crops) land use systems. Intensive systems caused significant ( t-test, P < 0.05) SOC loss of 10.3% or 6.74 Mg ha −1 in the 0–20 cm depth, but not in the 0–40 cm depth. In general, non-intensive systems had no significant effect on SOC stocks in the 0–20 and 0–40 cm depths. However, in coarse-textured soils (≤200 g clay kg −1), non-intensive systems caused significant SOC losses of about 20% for both 0–20 and 0–40 cm depths (8.5 and 15.5 Mg SOC ha −1, respectively). No significant changes in SOC were detected in Oxisols (<500 g clay kg −1) and clayey Oxisols (≥500 g clay kg −1), in five arbitrary ecoregions or under the three different non-intensive land use systems. Average SOC losses under intensive and non-intensive systems were low in comparison to those reported for temperate ecosystems, probably due to: (a) lower SOC stocks in surface when compared to temperate soils, and (b) strong interaction of Al/Fe oxides in clay with SOC.