Soil organic matter and CO₂ fluxes in small tropical watersheds under forest and cacao agroforestry

<div><p>Annual estimates of CO₂ and dissolved carbon concentrations in the soil profile provide valuable insight into the dynamics of organic matter in soil and the effect of changes to vegetation cover. The aim of this study was to observe the spatial influence of litter decomposition in the first few centimeters of the soil for CO₂ fluxes and to describe the processing of soil organic matter throughout the soil profile by comparing three small tropical watersheds. Data were collected biweekly for six months, from December 2015 to May 2016. CO₂ was measured using an infrared gas analyzer in fixed chambers and the dissolved carbon of soil solution was analyzed in a TOC analyzer. No differences were found in the total soil CO₂ fluxes (control flux treatments) between the three study areas. In both cacao agroforestry systems (managed and unmanaged), total CO₂ fluxes were influenced by the decomposition of litter. CO₂ emissions in the soil profile of the cacao agroforestry systems were highly variable, compared to the preserved forest, and highly dependent on the soil characteristics attributed to the type of vegetation cover. Although a definite pattern between the temperature and soil moisture was not identified, these parameters showed a strong relationship in controlling the release of CO₂ between treatments. The organic and inorganic dissolved carbon patterns in the soil solution of the three areas revealed different responses of soil organic matter processing related to soil characteristics and vegetation. The results confirm the hypothesis that the top of soils (total CO₂ fluxes) of both cacao agroforestry systems (managed and unmanaged) emits fluxes of CO_2, which do not differ statistically from the preserved forest. However, depending on the soil characteristics, the cacao agroforestry system can result in an accumulation of CO₂ and dissolved inorganic carbon in the soil profile that is prone to being transported by hydrological routes to groundwater and stream water.</p></div

Location map of the small watersheds.

<p>(PF) Preserved forest, (MC) Managed cacao agroforestry system and (UC) Unmanaged cacao agroforestry system.</p

Principal component analysis (PCA) in monthly timescale determined by Axis 1 and Axis 2.

<p>(CO₂) CO₂ fluxes from the 20 cm treatment in mg CO2—C m² h^-1, (PREC) Precipitation in mm, (TEMP) temperature in °C, (MOISTURE) Soil moisture in %, (PF) Preserved forest, (MC) Managed cacao agroforestry system and (UC) Unmanaged cacao agroforestry system, (1, 2, 3, 4, 5 and 6) December, January, February, March, April and May, respectively.</p

Principal component analysis (PCA) in monthly timescale determined by Axis 1 and Axis 2.

<p>(CO₂) CO₂ fluxes from the 40 cm treatment in mg CO2—C m² h^-1, (PREC) Precipitation in mm, (TEMP) temperature in °C, (MOISTURE) Soil moisture in %, (PF) Preserved forest, (MC) Managed cacao agroforestry system and (UC) Unmanaged cacao agroforestry system, (1, 2, 3, 4, 5 and 6) December, January, February, March, April and May, respectively.</p

Treatments for CO₂ measurements.

<p>(CF) Controls flux, (F20) Flux of 20 cm, (F40) Flux of 40 cm.</p

Spatial variation of the CO₂ fluxes (mg CO₂—C m² h^-1).

<p>(CF) Control fluxes, (F20) chambers with 20 cm and (F40) chambers with 40 cm, (PF) Preserved forest, (MC) Managed cacao agroforestry system and (UC) Unmanaged cacao agroforestry system. Different letters indicate different values for the statistical Mann-Whitney <i>U</i>-test, with a <i>p</i> < 0.05.</p

Characteristics of small watersheds.

<p>Characteristics of small watersheds.</p

Soil moisture (%) of the small watersheds.

<p>(A) 10 cm depth to Preserved forest—PF <i>n =</i> 32, Managed cacao agroforestry system- MC <i>n</i> = 33, Unmanaged cacao agroforestry system—UC n = 42 and (B) 20 cm depth for Preserved forest—PF <i>n =</i> 28, Managed cacao agroforestry system—MC <i>n</i> = 26, Unmanaged cacao agroforestry system—UC n <b>=</b> 30.</p

Soil temperature (°C) of the small watersheds.

<p>(A) 10 cm depth to Preserved forest—PF <i>n =</i> 53, Managed cacao agroforestry system- MC <i>n</i> = 55, Unmanaged cacao agroforestry system—UC n <b>=</b> 57 and (B) 20 cm depth for Preserved forest—PF <i>n =</i> 40, Managed cacao agroforestry system—MC <i>n</i> = 41, Unmanaged cacao agroforestry system—UC n <b>=</b> 39.</p

Spatial variation of dissolved inorganic carbon–DIC–(mg L^-1) in soil solution (SS) at depths 15 cm (SS15), 45 cm (SS45) and 90 cm (SS90).

<p>(PF) Preserved forest n = 8, 16 and 13, (MC) Managed cacao agroforestry system n = 6, 9 and 9 and (UC) Unmanaged cacao agroforestry system n = 5, 5, 5 for SS15, SS45 and SS90 respectively. Different letters indicate different values for the statistical Mann-Whitney <i>U</i>-test, with <i>p</i> < 0.05.</p