The nitrous oxide (N 2 O) emitted from soil was monitored to investigate the effect of sugarcane straw removal on the mechanisms that make mulch a Bhot spot^of N 2 O emissions under subtropical conditions. We conducted a field experiment with the firstratoon sugarcane with four amounts of straw (0, 4, 8, and 12 Mg ha −1) at the soil surface combined with 0 or 100 kg urea-N ha −1. The urea-N was applied 52 days after straw application. Over the course of 1 year, we measured the N 2 O and carbon dioxide (CO 2) emissions, mineral nitrogen (N), soil moisture and temperature, and remaining straw carbon (C) and N in the mulch. We observed two Bhot moments^for N 2 O emissions: the first one immediately after sugarcane straw application to soil and the second one after fertilizer-N application. High amounts of straw left on the soil led to an increase in the water-filled pore space (WFPS), and both WFPS and straw-C were strongly correlated with N 2 Ofluxes.CumulativeN 2 O increased from 510 (0 Mg + N) to 1055 (12 Mg + N) g N 2 ON ha −1 for the fertilized straw treatments. The N 2 O emission factors (EFs) of the sugarcane straw N and the fertilizer-N increased linearly with straw quantity, i.e., were not constant but were lower than the IPCC default values. Over 70% of the cumulative N 2 O emissions measured in straw + fertilizer-N treatments for 1 year were attributed to the presence of straw mulch, which emphasized the importance of the straw layer at the soil surface as a hot spot for N 2 O emissions.
The increasing demand for meat and milk has stimulated interest in the development and recovery of pastures. Although humic acids (HAs) have been shown to have a biostimulating effect on plant growth, their use in pastures remains relatively unknown. This study aimed to evaluate the growth of Brachiaria decumbens at different HAs concentrations and application times. This study was conducted in a greenhouse with pots containing 1.5 L of soil collected in a degraded pasture. A trial was carried out in a randomized block design in which five HAs concentrations (0, 7.5, 15, 30 and 60 mg L-1 C) and four application times were used as follows: leaf spray 15 days after emergence (LS 15 DAE); LS 45 DAE; LS 60 DAE; and successive leaf sprays (15, 45 and 60 DAE). Stem height, diameter, forage production and root development were evaluated. Height and stem diameter measurements were performed weekly. At 45 and 90 DAE, a cut was made at a height of 10 cm to simulate grazing and to measure forage production. At 90 DAE, the root mass was assessed. The HAs used at the concentration of 60 mg L-1 C applied at 15 DAE promoted increases of 44% in plant height and 196% in forage mass. After the cut, the optimum concentration was approximately 40 mg L-1 C. Although the use of HAs promoted root development, successive applications had deleterious effects on the plant. HAs could improve pasture biomass production at a low cost if applied at the ideal concentration.
Recovery of degraded pastures improves the sustainability of meat and milk production systems. Humic acids (HA) and endophytic diazotrophic bacteria (EDB) are known to have bio-stimulating effects on several crops but have not been tested for the recovery of degraded pastures. The aim of the present study was to evaluate soil C and N dynamics, nutrient uptake and forage yield of Brachiaria decumbens following the foliar application of HA and EDB in a degraded pasture. A randomized block design with six replications was used. The four treatments were: HA, EDB, HA+EDB and Control. The subplots consisted of four evaluation times for soil characteristics: before treatment application (0) and 30, 60 and 120 days after treatment application (ATA). Soil total organic carbon (TOC), recalcitrant carbon (Crecal), labile carbon (Clabil), total nitrogen (TN) and C and N stocks were determined in the 0-5, 5-10, 10-20 and 20-40 cm of soil layers. Forage yield and leaf nutrient contents were evaluated at 30 days ATA. The application of the HA and EDB increased the TOC, Clabil, soil C stock (≈18%), forage dry matter yield (16 to 52%) and nutrient uptake (≈30%) after 30 days ATA. Our results showed that the application of HA combined with EDB may be a strategy for the recovery of degraded pastures in the tropical region.
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