Soil C, N and P
The presence of trees, as compared to pasture, caused larger topsoil C and N contents in the areas affected by the tree, i.e. under the canopy and in the litter cone, while P was larger in the litter cone. In the subsoil, C content was also larger due to tree presence. In addition, the interaction between litter deposition and tree species showed larger C and N contents under Jícaro in both the top- and subsoil. The trees also affected soil stoichiometry. In the topsoil, lower C:N ratios point at larger tree induced N availability, while higher C:P and N:P ratios suggest depletion of P. In the subsoil, P related stoichiometry showed P depletion under Jícaro.
The larger tree-induced soil C and N contents may be due to the higher net primary productivity and leaf litter production of systems with trees as compared to pasture vegetation only, because multi-strata systems have higher resource uptake and photosynthetic capacity (Chapin et al. 2012). Working in SPS in the Matagalpa district in central Nicaragua, Casals et al. (2014) also observed soil C and N contents to be larger under trees as compared to open pasture and, based on δ13C analysis, attributed this to higher above- and belowground litter inputs from trees. Also, based on soil profile δ13C values in SPS in Florida, USA, Haile et al. (2010) were able to show that trees (C3) contributed more C to the silt + clay-sized fraction (<53 µm, which is comparable to the maHF in this study) than C4 grasses. This tree effect increased with soil depth.
As mentioned, soil P content was observed to be larger in the litter cone area as compared to pasture, while P depletion occurred underneath Jícaro. Obviously, P was taken up by Jícaro trees from underneath the canopy and subsequently released by leaf litterfall into the litter cone area. This mechanism may create differences in soil P content at relatively short distances (3–5 m), i.e. underneath the canopy versus litter deposition area. Working with isolated trees in a comparable Dehesa system, Gallardo (2003) also observed that isolated trees differentially affect the spatial distribution of nutrients, with P having a larger spatial range (beyond the canopy) as compared to N (within canopy projection). In this study however, this spatial differentiation was less pronounced for Guácimo trees, where, despite soil P enrichment in the leaf litter cone was observed, significant P depletion, based on higher C:P and N:P ratios in the canopy subplots, was not detected. Casals et al. (2014) found higher soil P concentrations under the canopies of trees, including G. ulmifolia (Guácimo), in Central Nicaragua. Also, Martínez et al. (2014) working with SPS in Colombia observed higher available soil P levels in plots associated with trees, including G. ulmifolia (Guácimo), as compared to plots without trees. However, the results of these studies may still be in line with our study for Guácimo plots, when the spatial differentiation between P uptake and P input through litter deposition is less pronounced than in the Rivas area. The prevailing north-easterly winds coming across Lake Nicaragua caused a spatial differentiation between P uptake and deposition, which may be negligible in other areas. Still, taking spatial differences in account, trees enhance P cycling and available soil P stocks.
Soil organic matter fractions
A small part of the relatively fresh and unprotected OM (fLF) may become incorporated into aggregate structures (oLF) or become adsorbed onto mineral surfaces (maHF), while most of it will be decomposed and respired with C leaving the soil as CO2 and N and P being mineralized into plant available forms. Tree presence and its litterfall increased labile C, N and P fractions (fLF) in the topsoil, which allows higher N and P fertility and larger input of OM to the occluded and mineral associated fractions (SOM stabilization). In the subsoil, only fLF P was affected with a larger labile P fraction under Guácimo. This species-induced difference is in line with the larger uptake of available P by Jícaro as discussed above.
By increasing free light C, N and P fractions (fLF), tree presence also resulted, through increased soil fauna and microbial activity, in more soil aggregation and larger occluded light C, N and P fractions. Also the larger P fLF due to Guácimo resulted in a larger P oLF under Guácimo, which may be due to higher litter P content of this tree or, as suggested above, relatively stronger P uptake by Jícaro. Differences in plant traits, i.e. leaf litter quality, may also have resulted in larger subsoil oLF N in the litter cone subplots of Jícaro.
During further decomposition of either free or occluded OM, relatively smaller organic molecules or even soluble organic C may become adsorbed onto mineral surfaces (Saidy et al. 2015; Von Lützow et al. 2006). The tree-induced larger free and occluded light C, N and P fractions resulted in larger stabilized C and P fractions in the topsoil for both trees species, while stabilized N was enhanced under Jícaro. As for the whole soil N and P contents and the free and occluded N and P fractions, Jícaro enhanced the stabilized N fraction, while Guácimo enhanced stabilized P fractions at both soil depths. Mineralization and nutrient uptake by trees may have caused the somewhat lower C maHF in the canopy subsoil.
The larger CO2 efflux in the leaf litter subplots as compared to underneath the canopy can obviously be attributed to the larger leaf litter input causing increased heterotrophic respiration. However, in the canopy subplots we expected some extra CO2 efflux by autotrophic tree root respiration, but obviously did not make up for the leaf litter effect. Also, lower soil temperatures underneath the canopy may have suppressed CO2 efflux. Although the respiration rates in the pasture subplots were lower than in the leaf litter subplots, the difference was not significant, which suggests that part of the extra C input to the leaf litter subplots may stay in the soil. The latter is supported by the whole soil and fractionation data that show that tree presence enhanced soil C content.
Role of trees
Our hypothesized role of trees in SPS was primarily confirmed by the effect of tree litter deposition causing topsoil C, N and P content to be larger in the canopy and litter cone subplots as compared to open pasture. Moreover, soil fractionation showed that tree-induced larger litter input subsequently increased free and occluded OM fractions and ultimately increased stabilized SOM fractions. Therefore, trees enhance soil C sequestration in these SPS. Casals et al. (2014) found similar results and also Haile et al. (2010) suggested that pastures with trees have greater potential to sequester more soil C as compared to treeless pastures. Although in colder ecosystems, in a review of studies in temperate and boreal forests Vesterdal et al. (2013) also found tree species effects on SOC stocks.
Both tree species enhanced nutrient cycling but had different effects on soil N and P, where Jícaro enhanced available and stabilized N and Guácimo enhanced available and stabilized P. The study by Casals et al. (2014) included a larger diversity of tree species, but they concluded that the presence of trees increased soil C and fertility and that the magnitude of this effect depends more on tree size than on species traits like being leguminous or not. However, our results show a clear tree species effect on nutrient cycling which is most likely due to differences in functional traits (Augusto et al. 2015). From a management point of view, this and related studies clearly show that tree presence in these SPS contribute to soil C sequestration and nutrient availability. More specifically, the prevailing wind in the Rivas area and the resulting litter deposition causes differences in available organic N and P fractions at short distances. Moreover, choice between Jícaro versus Guácimo may enhance respectively soil N and P availability.