Sistemas silvipastoris com leguminosas arbóreas e Brachiaria decumbens Stapf : biomassa arbórea, decomposição de folhas e fertilidade do solo

Abstract

Forestry systems integrating tree legumes can benefit the sustainability of the ecosystem, promoting environmental services. In this sense, the objectives were evaluate the productive and chemical potential of tree legumes, compare three methodologies for decomposing senescent legume leaves, analyze the decomposition in the soil of senescent leaves of Gliricidia sepium (Jacq.) Steud. (Gliricidia), Mimosa caesalpiniifolia Benth. (Sabiá) and Braquiaria decumbens Stapf. (Signalgrass), and evaluate soil fertility in silvopastoral systems. The experimental plots consisted of the consortia Signalgrass + Gliricidia, Signalgrass + Sabiá and Signalgrass in monoculture. The experimental design was randomized in blocks, with three replications, in plots maintained under continuous stocking and variable stocking rate. Four samplings were carried out between October/16 – April/18 (6-month intervals), to evaluate dendrometric variables, total biomass (BT) of legumes [leaves up to 1.5 m high (L15) and total (LT), and thin branches (Tnb, <12.9 cm in circumference), intermediate (Ib, 13–20 cm) and thick (Tcb, >20 cm)]; chemistry of legume and Signalgrass leaves [dry matter (DM), organic matter (OM), nitrogen (N), acid detergent fiber (ADF), lignin and nitrogen insoluble in acid detergent (ADIN)] and in legumes, carbon:nitrogen ratio (C:N), natural abundance of 15N (δ15N), 13C (δ13C) and nitrogen derived from the atmosphere (Ndfa). In the comparison of leaf decomposition processes, three incubations were performed, being in the soil (0, 4, 8, 16, 32, 64, 128 and 256 days) with nylon and non-woven bags (NWB) and in the rumen (0, 6, 12, 24, 48, 96 and 144 hours) with NWB Parameter estimation models were developed for further analysis considering the incubation total time percentage. In the soil (nylon bags), the decomposition processes of legumes and Signalgrass were compared. In the soil, the physical and chemical properties [pH, phosphorus (P), potassium (K+), calcium (Ca+2), magnesium (Mg+2), sodium (Na+), exchangeable aluminum (Al+3) and potential acidity (H++Al+3), estimating the exchangeable bases sum (EBS), effective and potential cation exchange capacity (CEC), base saturation (V) and Al+3 saturation (m)], and biological [organic matter light fraction (MOLF), basal respiration (SBR) and natural abundance (δ13C) of the breathed CO2]. Soil chemical analyzes in 2013 were also considered. Data were analyzed using SAS University. The variables base diameter (6.0 and 7.6 cm), stem length (5.5 and 7.5 cm), Lorey height (5.9 and 7.2 cm), tree volume (0, 2 and 0.3 m3), Ib (7.5 and 13.9 Mg DM·ha-1), TL (1.8 and 3.0 Mg MS·ha-1), DM (252 and 427 g·kg-1 as a fed), lignin (145 and 233 g·kg DM-1), ADIN (168 and 272 g·kg N-1) suffered species effect with higher averages in Sabiá (P<0.05). While, Gliricidia was superior in CP (250 and 187 g·kg DM-1), δ13C (-34 and -35 ‰) and Ndfa (65 and 47%). Decomposition rate (k) of N from Sabiá and lignin, C:N and lignin:N from Gliricídia were higher (P<0.05) in the soil, regardless of textile used. In assessing the decomposition process in the soil, k was superior (P<0.05) for Signalgrass in DN, C and C:N ratio, for Gliricidia and Signalgrass in organic matter (OM) and for Gliricidia in N. In the soil, pH ( 5.3; 5.2; 5.1), P (11.3; 7.2; 3.6 mg·dm-3), Mg+2 (2.6; 1.1; 1.5 cmolc·dm-3), K+(0.23; 0.17; 0.12 cmolc·dm-3), EBS (5.3; 4.6; 4.6 cmolc·dm-3) and CECeffective (5.8; 5.1; 5.0 cmolc·dm-3) suffered a reduction (P<0.05), while H++Al+3 (4.97; 9.6; 9.8 cmolc·dm-3) suffered elevation (P<0.05), respectively for the years 2013, 2017 and 2018. Soil basal respiration suffered reduction (0.21–0.14 mg CO2·kg soil-1·h-1) in relation to the depth 0-20 and 20-40 cm, respectively. Sabiá demonstrated continued growth and potential to incorporate nutrients into the soil. Gliricídia provides forage material and has greater potential for NBF. Dynamics of senescent leaves decay of Gliricídia and Sabiá in the soil can be evaluated by means of nylon or TNT bags. Gliricidia has a higher rate of leaf decomposition, with the potential for rapid release of nutrients to the soil; while Sabiá shows slower release of nutrients, however, with the potential to form a more stable OM. Decomposition rate of Signalgrass is limited by the low N content of leaves, providing a high C:N ratio and immobilization of N during the process. Silviculture systems intercropped with Signalgrass, without maintenance fertilization, maintain uniformity in the physical properties of the soil, although there is a reduction in fertility. Both systems contribute to the sequestration of C.