Tree litter quality more than nutrient availability drives the soil carbon stabilization under lab conditions

Eucalyptus ssp. (eucalyptus) is commonly used in integrated crop-livestock-forestry and livestock-forestry systems. Acacia mangium (acacia) grows as fast as eucalyptus in tropical conditions and is capable of fixing N₂ from the atmosphere. Both species differ in terms of litter quality: eucalyptus has a higher C:N ratio, lower N:P and C:P ratios, and lower lignin and cellulose contents compared to acacia. This study evaluates how the residues (litter) of each species affect the C and N contents (in g kg^-1) in particulate organic matter (POM) and mineral-associated organic matter (MAOM) when nutrients (N, P, S) that intensify the humification process are applied to the soil under two different levels. Our hypothesis is that the sequestration of C-rich litter into the fine fraction (MAOM) could be improved by adding supplementary nutrients. The experiment was carried out in the laboratory (incubator at 30^oC, in the dark). Pots were prepared with 200g of soil and 3,82 g of litter. A nutrient solution was prepared to correct the added litter simulating the stoichiometric C:N:P:S ratios of humus (~10,000:833:200:143). The solution was added to the pots in dosages equivalent to 0x, 1x and 2x the calculated amount. After 180 days of incubation, N content in POM (g kg^-1) was 113% larger in pots with acacia residues (0.257) compared to eucalyptus (0.121) (p < 0.05). Similarly, the C content in POM was 82% larger for acacia (7.63 g kg^-1) vs. eucalyptus (4.20 g kg^-1). No significant effects were observed for C and N in POM among nutrient dosages. Contrarily, in MAOM, N and C were significantly larger (p<0.001 and p<0.05, respectively) in pots with eucalyptus (3.08 and 67.6 g kg^-1) vs. acacia residues (2.82 and 65.2 g kg^-1). The N content in MAOM was larger (p<0.05) under high nutrient dosages (2x) compared to 0x and 1x dosages. However, C content did not differ among nutrient dosages. We conclude that the recalcitrance of acacia residues favors N and C storage in POM. On the other hand, N and C are enhanced in MOAM fostering long-term sequestration when eucalyptus residues are present. Although the tree species affect most the humification process (and C stabilization), fertilization studies respecting humus stoichiometry must be carried out at field level, due to greater efficiency of formation mineral-stabilized soil carbon by the rhizosphere microbial communities relative to the bulk soil community.