Pasture degradation impacts soil phosphorus storage via changes to aggregate-associated soil organic matter in highly weathered tropical soils

Maintaining the productivity of tropical pastures is a major challenge for the sustainable management of tropical landscapes around the globe. To address this issue, we examined linkages between soil organic matter (SOM), aggregation, and phosphorus (P) dynamics by comparing productive vs. degraded pastures in the deforested Amazon Basin of Colombia. Paired plots of productive (dominated by planted Brachiaria spp.) vs. degraded pasture were identified on nine farms in the Department of Caquetá and sampled during the rainy season of 2011. Aboveground pasture biomass production and nutrient content were measured. Surface soils (0-10 cm) were also fractionated by wet sieving, and C, 13C, N and P contents were analyzed for the bulk soil and various aggregate size classes. Productive pastures yielded more than double the aboveground biomass compared to degraded pastures (during a 35 day regrowth period following cutting), with over 60% higher N and P contents in this material. Similar trends were observed for the standing litter biomass and nutrient contents. Soil aggregate stability was found to differ between pasture types, with a mean weight diameter of 3590 vs. 3230 μm in productive vs. degraded pastures, respectively. Productive pastures were found to have 20% higher total soil C and N contents than degraded pastures. While there was no difference in total P content between pasture types, organic P was found to be nearly 40% higher in soils of productive vs. degraded pastures. Differences in total SOM between pasture types were largely explained by a higher C content in the large macroaggregate fraction (>2000 μm), and more specifically in the microaggregates (53-250 μm) occluded within this fraction. These findings confirm the role of microaggregates within macroaggregates as a preferential site for the physical stabilization of SOM, and furthermore, suggest that it may serve as a useful diagnostic fraction for evaluating management impacts on SOM in tropical pasture systems. Similar to trends observed for C and N, total P content was 25% higher in the microaggregates within large macroaggregates of productive vs. degraded pasture soils. This correspondence between C and total P contents in large macroaggregate fractions, along with elevated levels of organic P in productive pastures, suggests that this P is likely in an organic form and that there is a close link between soil structure, SOM dynamics and the maintenance of organic P in these soils. Given the potential relevance of organic P for efficient P cycling in these soils, our findings offer critical new insight for the management of SOM and aggregate-associated P pools in tropical pasture systems.