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

Background

Land disturbance affects soil physical and chemical properties. Some properties may be recovered over long periods of reforestation. Mosaic-pattern landscapes with shifting usages over time, common in the mountainous tropics, can reveal dynamic soil properties. This study reports on changes in soil carbon over 80 years of secondary forest growth on abandoned pasture over a chronosequence in Puerto Rico.

Research goals & methods

The study combines a chronosequence approach with differences in the natural abundance of 13C between forest and pasture plants to estimate turnover times of carbon in the bulk soil and in density fractions. Overall, gains in secondary forest carbon were compensated for by the loss of residual pasture-derived soil carbon, resulting in no net change in bulk soil carbon stocks down to 1m over the chronosequence.

Turnover times varied with model choice, but in general, soil C cycling rates were rapid for the 0–10 cm depth. The majority of the soil carbon pool was in the mineral-associated density fraction. Carbon-to-nitrogen ratios and differences in natural abundance of soil organic matter (SOM) showed an increasing degree of decomposition across density fractions with increasing mineral association.

Conclusions & takeaways

The loss of soil carbon during cultivation is well documented, but pastures may have negative, positive, or net zero effects on soil carbon. These data show that the bulk soil carbon pool was maintained during conversion to pasture and over 80 years of reforestation. The study suggests that the physical distribution of carbon in the soil has a large impact on soil carbon turnover and the ability of soils to maintain SOM stocks during land-use and land-cover change.