C and N stocks in silvopastoral systems with high and low tree diversity: Evidence from a twenty-two year old field...

This paper aimed to quantify the C and N stocks and the natural abundancy of ¹³C (δ¹³C) in organic matter fractions of soil, as well as soil labile C, in a long-term experiment (1997-2019) on silvopastoral systems (SSP) with low and high tree diversity in the Mata Atlântica biome. Disturbed and undisturbed soil samples were collected in transects that were perpendicular to the tree rows every five meters (0, 5, 10, and 15 m of distance), at depths of: 0.0-0.10, 0.10-0.30. 0.30-0.60, 0.60-1.0 m. Litterfall, root density, total organic C and N content, δ¹³C values for particulate organic matter (POM) fractions as well as mineral-associated organic matter (MAOM) and soil labile C, whilst stocks were calculated afterwards. Litterfall production was higher in the high diversity SSP for all distances, except for 5 m from the tree row. In contrast to litterfall, higher root density was observed for the longest tree distances (15 m) in the low diversity system. The high tree diversity SSP increased TOC stocks in the top soil layer (0.0-0.10 m, distances of 0 and 10 m) only and C-MAOM stocks in the surface (0.0-0.10 m, 10 m distance) and subsurface (0.10-0.30 m, 0 m distance). In contrast, total N stocks or stocks associated with MAOM and POM were higher in the high tree diversity system. The increases were of 37, 36 and 63%, respectively, for total N, N-MAOM, and N-POM up to 1 m depth. The smallest δ¹³C values found close to the tree row (0 and 5 m distances), especially in the high diversity system, indicate the influence organic residue from leguminous tree species, whilst the C4 grass held the highest contribution at the 15 m distance, also in the high diversity system. In general, the high diversity tree silvopastoral system has shown potential at storing most C in the 0.0-0.10 m soil layers only and N up to 1 m depth. Results showed very little shift in total C stocks, but the increased N stocks with the inclusion the legumes trees, which is reflected in a lower C:N ratio of the SOM. The silvopastoral system containing signal grass both legumes trees can be looked at as a viable strategy towards sustaining existing soil C stocks, whilst increasing N stocks and SOM quality.