Soil organic matter dynamics during 80 years of reforestation of tropical pastures

Abstract

Our research takes advantage of a historical trend in natural reforestation of abandoned tropical pastures to examine changes in soil carbon (C) during 80 years of secondary forest regrowth. We combined a chronosequence approach with differences in the natural abundance of 13 C between C3 (forest) and C4 (pasture) plants to estimate turnover times of C in the bulk soil and in density fractions. Overall, gains in secondary forest C were compensated for by the loss of residual pasture-derived soil C, resulting in no net change in bulk soil C stocks down to 1 m depth over the chronosequence. The free light fraction (LF), representing physically unprotected particulate organic matter, was most sensitive to land-use change. Reforestation replenished C in the free LF that had been depleted during conversion to pastures. Turnover times varied with model choice, but in general, soil C cycling rates were rapid for the 0-10 cm depth, with even the heavy fraction (HF) containing C cycling in decadal time scales. Turnover times of C in the free LF from the 0-10 cm depth were shorter than for the occluded and HFs, highlighting the importance of physical location in the soil matrix for residence time in the soil. The majority of the soil C pool (82 AE 21%) was recovered in the mineral-associated density fraction. Carbon-to-nitrogen ratios and differences in natural abundance 15 N of soil organic matter (SOM) showed an increasing degree of decomposition across density fractions with increasing mineral association. Our data show that the physical distribution of C in the soil has a large impact on soil C turnover and the ability of soils to maintain SOM stocks during land-use and land-cover change.

Carbon dynamics of soil organic matter in bulk soil and aggregate fraction during secondary succession in a Mediterranean environment

“…In the silt-clay aggregate fraction, however, new and old carbon were nearly equivalent 60 years after abandonment. According to Marin-Spiotta et al (2009), the organic carbon in macro-aggregates and microaggregates are relatively unprotected and are consequently more sensitive to the influence of vegetation dynamics after land abandonment. The current results demonstrate that the SOC sequestration potential is lower for C 4 than for woody C 3 plant communities, which is due to the lower primary productivity and the high lignin content of C 4 H. hirta Table 4 SOC (g C kg −1 bulk soil) and δ 13 C in bulk soil and fractions in the seven terraces of the chronosequence at 15-30 cm depth.…”

Comparison of soil organic matter dynamics at five temperate deciduous forests with physical fractionation and radiocarbon measurements

“…Chemical analysis of density fractions has shown an increasing degree of decomposition with increasing degree of mineral association across density fractions (Marín-Spiotta et al 2009;Golchin et al 1994). Previous studies have also shown that fLF tends to consist of plant materials with a visible structure, sometimes coated in mineral particles ); while oLF tends to contain highly fragmented plant debris, char, pollen and fungal spores, and unrecognizable, partially decomposed organic matter Golchin et al 1994).…”

Past and present land use influences on tropical riparian zones: an isotopic assessment with implications for riparian forest width determination

“…Differences between the present study and the one by Vitorello et al (1989) might be related to soil characteristics such as texture (Roscoe et al 2001) or other factors affecting soil carbon turnover rate (Powers & Schlesinger 2002;Telles et al 2003) such as soil moisture that might be typically higher in riparian soils compared to upland soils (Luizã o et al 2004). Moreover, the long-term persistence of C 3 signals in the riparian areas of watersheds 1 an 2 might be attributed to highly recalcitrant organic matter or organic matter that is tightly bound to clay soil particles (Roscoe et al 2001, Powers & Schlesinger 2002, Telles et al 2003, Alcâ ntara et al 2004, Marin-Spiotta et al 2009). …”

“…The soil d 13 C in this area (5m-transect) does not reflect the d 13 C of the forest, but instead the d 13 C of a C 4 vegetation, suggesting that this forest is a young secondary growth that did not yet contributed substantially to the soil organic matter (Marin-Spiotta et al 2009) or that upland C 4 influences within this area are substantial due to C 4 organic matter deposition (Pires et al 2009). The soil d 13 C of the 15 m and 30 m-transect lines reflect the C 4 vegetation type.…”

“…The younger the riparian forest, the weaker would be the C 3 signal in soil organic matter (Marin-Spiotta et al 2009). Provided that most of riparian forests presented here are riparian forest remnants (watersheds 1, 4 and 5), no in situ influence (i.e.…”