Nodulation and biological nitrogen fixation (BNF) in forage peanut (Arachis pintoi) cv. Belmonte subjected to grazing regimes

Productivity of grassland ecosystems is highly dependent on the availability of nitrogen (N) from the soil and/or biological fixation (BNF) from the atmosphere. However, the high cost of N-fertilizers and the increasing awareness of the need for sustainable nutrient alternative sources have increased the strategic importance of the BNF. Defoliation interferes with plant growth and also with BNF in legumes, since it modifies the leaf area and the availability of energy to plants. In spite of the potential advantages of forage legumes to pastures, current knowledge regarding the effects of grazing management on BNF is scarce. In order to evaluate the effect of grazing management on aerial and root mass, nodulation and BNF in forage peanut (Arachis pintoi cv. Belmonte), two experimental protocols based on the continuous and intermittent (rotational grazing) stocking methods were used. In the continuous stocking protocol, treatments corresponded to four grazing intensities represented by four management heights - 5, 10, 15 and 20 cm. In the intermittent stocking protocol, treatments corresponded to combinations between two pre- (95% and maximum canopy light interception during regrowth – LI95% and LIMax, respectively) and two post-grazing (post-grazing heights equivalent to 40 and 60% of the pre-grazing height) conditions. It was hypothesized that hard grazing, represented by low management heights under continuous stocking or low post-grazing heights under intermittent stocking, causes reduction in root mass and nodulation interfering with the balance between root and aerial, with negative impacts on BNF. Under continuous stocking, greater number of nodules was associated with lower nodule mass and vice-versa throughout the year. Swards managed at 5 and 10 cm had greater root and nodule mass than those managed at 15 and 20 cm. However, greater BNF (%Ndfa and BNF) was recorded on swards managed at 15 and 20 cm relative to those managed at 5 and 10 cm. Under intermittent stocking, the combinations between frequencies and severities of grazing did not affect root mass or nodule number at both pre- and post-grazing conditions. Overall, swards managed with the LIMax target showed greater BNF than those managed with the LI95% target. For swards managed with the same LI target (LI95% or LIMax), hard grazing (40% of the pre-grazing height) resulted in lower %Ndfa and BNF during early and late spring and summer II. For both grazing methods, the range of management targets used did not result in negative impacts on root mass or nodulation. Regardless of grazing method, BNF was determined by aerial biomass, indicating that moderate intensities of grazing would be adequate for optimizing BNF.