Trees enhance soil carbon sequestration and nutrient cycling in a silvopastoral system in south-western Nicaragua

Study area

This study was conducted in the municipalities of Belén and Rivas of the Rivas department in south-western Nicaragua (UTM 16P; E 620780; N 1271756). The mean annual temperature of the Rivas department is 27° C and the mean annual precipitation is 1400 mm (INETER 2000). The natural vegetation of the region corresponds to a transitional tropical sub-humid forest (Holdridge 2000) with semi-deciduous vegetation, and is referred to as seasonally dry tropical forest (Bullock et al. 1995), but the land has been widely modified over the past centuries and the present-day landscape of Rivas is dominated by pastures (56.7%) (Sánchez et al. 2004). Various SPS practices can be found in the Rivas area (e.g. hedgerows), but this study focused on isolated trees in pastures. Soils in the Rivas area were formed on marine parent material of young Tertiary age which consists of clays and sands of varying thickness. The Rivas complex has been lightly folded and eroded, resulting in a landscape with rolling to steep hills. Alluvial fans with slopes of less than 10% were formed as a result of erosion in the hills. These areas are largely heavy-textured and range from clay loam on the fringes of valleys to heavy clay in the central areas. The clays are largely montmorillonitic, causing the formation of Vertisols (greatgroup Haplusterts) which swell upon wetting and form deep cracks upon drying. Only suitable trees on flat parts of the landscape were selected to eliminate the confounding effects of slope, i.e. recent erosion and/or sedimentation, in our research plots.

Tree species

Guazuma ulmifolia, locally known as Guácimo, is one of the most common trees in SPS of the Department of Rivas and is also one of the most used species as a source of forage and food in the dry season (Sánchez et al. 2004). It is a non-leguminous species (Flores et al. 1998) of a maximum height of about 8 m in the open field. It loses its leaves during the end of the dry season, in February and March. Flowers and fruits are produced largely in the leafless period in the dry season, between January and April (Cordero and Boshier 2003; Francis, 1991). The selected trees were all full-grown (about 8 m height) and based on their stature were estimated to be over 50 years of age.

Crescentia alata, also known as Jícaro, is common in the Rivas SPS and was selected next to G. ulmifolia because both species are about similar in size and crown architecture. A database with individuals of both species was available with information on location, size, land owner, etc. C. alata is a species that is tolerant to extreme (very wet and very dry) conditions. It produces flowers and large gourd-like seeds along its stem. The species does not lose its leaves throughout most of the dry season and is therefore semi-deciduous. Only near the end of the dry season the leaves are dropped, but new leaves are produced shortly thereafter, before the start of the rainy season (Cordero and Boshier 2003; Rockwood 1974). Flowers and fruits are produced all year round and are used as feed supplements. Similarly, the selected trees were all full-grown (about 8 m height) and based on their stature were estimated to be over 50 years of age.

Experimental design

Next to being positioned on stable and relatively flat landscape positions, trees (plots) were also selected on the basis of having sufficient distance to other trees, i.e. at least 25 m, in order to avoid leaf litter deposition interactions. As mentioned, three sample areas per tree (subplots) were distinguished on the basis of environmental conditions and where leaf litter was deposited (Fig. 1): (1) open pasture—no tree litter deposition; (2) tree canopy—above and belowground tree litter; and (3) leaf litter cone—aboveground tree litter deposition. In each subplot, a soil pit of at least 50 cm depth was dug, in open pasture at least 10 m upwind from the tree crown perimeter; under the canopy, half way between the trunk and the projection of the crown perimeter; and centrally in the leaf litter cone, at least 3 m beyond the projection of the crown perimeter. Soil samples were taken at 0–20 and 20–40 cm soil depth between late August and early September of 2010. At each depth, three metal bulk density rings of 100 cm³ were inserted into the undisturbed pit face. In addition, next to the rings and at similar depth, soil was collected and bagged for soil fractionation and C, N and P analysis in whole soil and all soil fractions. Soil samples were placed in a cooler for transport to the laboratory. In total 216 bulk density samples and 216 bagged samples were taken (12 trees × 3 litter sample areas × 2 depths × 3 replicates).

Soil analysis

Field moist bulk density samples were weighed and dried in an oven at 105 °C for 24 h in the soil laboratory of the Escuela Internacional de Agricultura y Ganadería (EIAG) in Rivas. Soil bulk density was calculated by dividing the dry sample weight by 100 cm³ (core method; (Burt 2004). Bagged samples were air-dried at the laboratory in Belén, roots were picked out, samples were ground and homogenized and prepared for shipping to the laboratory at Wageningen University, The Netherlands.

The three bagged soil samples collected at each depth per subplot were mixed and homogenized yielding a total of 72 composite samples. After grinding, subsamples of the composited samples were analysed for total organic C and N with an element analyser (Interscience EA 1108). Organic and available P was determined according to Novozamsky et al. (1983). In short, samples were digested by subsequent additions of a selenium–sulphuric acid mixture and peroxide while heated to 330 °C (Gerhardt Kjeldatherm digestion system). After dilution of the digest, P was determined colorimetrically (spectrophotometer Mechatronics Starrcol SC-60-S at 720 nm).

Subsamples were subjected to density fractionation to obtain the following organic matter fractions: free light fraction (fLF), occluded light fraction (oLF) and mineral associated heavy fraction (maHF). Briefly, 10 g air dried soil <2 mm was shaken in a centrifuge tube with 50 ml sodium polytungstate (3Na₂WO₄·9WO₃·xH₂O; SPT) solution of density 1.6 g/cm³. The samples were centrifuged for 23 min at 4500 rpm (5900×g) (MSE Mistral 6000). The supernatant was poured over a Büchner funnel with a 0.7 µm Whatman GF/F glass filter using a Millipore vacuum filtration unit (Millipore, Bedford, MA). The light fraction was rinsed off the filter into a glass beaker and oven dried at 40 °C. Next, SPT solution was added to the residual soil material and shaken by hand to bring the precipitate into solution. An ultrasonic probe was used for 5 min to break the soil aggregates and to disperse the oLF. The suspension was centrifuged for 23 min at 4500 rpm. The centrifuged tubes were decanted over a Büchner funnel with a 0.7 µm Whatman GF/F glass filter. The oLF fraction was rinsed off the filter into a glass beaker and oven dried at 40 °C. The remaining precipitate, the maHF fraction, needed to be rinsed thoroughly in order to remove all STP. Total C, N and P contents of each fraction were determined similarly as for the whole soil samples.

Field soil respiration measurements were done between August 3–September 6 and October 20–November 2, 2010 (PP Systems, SRC-1 soil respiration chamber and EGM-4 CO₂ monitor and temperature probe). Time series of soil respiration (μmol CO₂/m²/s) and temperature (°C) based on hourly measurements between approximately 1 and 5 pm were determined in each subplot (open pasture, tree canopy, leaf litter cone). These CO₂ soil efflux measurements included root respiration and respiration due to microbial decomposition of litter and SOM. The average of each time series was used for further statistical analysis.

Statistical analysis

All data were plotted (histograms and Q–Q plots) and examined for outliers and normality (Field 2014). No data needed to be excluded or transformed. Because the subplots are not independent (affected by the same tree), a mixed design ANOVA was used with within-subjects factor “litter deposition” (3 levels) and between-subjects factor “tree species”. For each soil depth increment, the mixed ANOVA was implemented in IBM SPSS Statistics 22 by defining a Repeated Measures General Linear Model (Field 2014). Main or interaction effects were considered to be significant when the P value of the F-test was <0.05. Significant main or interaction effects are presented as footnotes to the respective figures and tables.