Nitrogen utilization and cycling in forage production systems

Abstract

Nitrogen cycling is strongly affected by each factor of the pasture system, including the grazing animals, insects, the grazing intensity or clipping stubble height, harvest frequency, and forage varieties. The aim of this study was to evaluate nitrogen utilization and cycling in different forage production systems. Specific objectives included (i) to evaluate different species of dung beetles and their assemblages on GHG emission (N2O), ammonia volatilization, and pearl millet [Pennisetum americanum (L.) Leeke] performance; (ii) to evaluate herbage accumulation (HA), crude protein (CP), nitrogen yield (Ny) and in vitro digestible organic matter (IVDOM) concentration of new Bermudagrass cultivars and (iii) to evaluate HA, Ny, biological nitrogen fixation (BNF) and nitrogen derived from atmosphere (%Ndfa), legume contribution (LC), dry matter (DM), CP, and IVDOM of Alfalfa-bermudagrass mixtures under contrasting harvesting regimes. All trials were allocated in a randomized complete block design (RCBD), each one with different treatments as (i) singles species as Onthophagus taurus (1), Digitonthophagus gazella (2) and Phanaeus vindex (3), and their assemblages combining species 1+2 and 1+2+3. In addition, two controls treatments were used; (ii) 10 bermudagrass genotypes, ‘Missouri’, ‘Tifton 85’, ‘Jiggs’, ‘FL44’, ‘322’, ‘323’, ‘276’, ‘282’, ‘283’, ‘286’and (iii) two alfalfa varieties which Bulldog 805 and UF2015-AP, clipped at 5, 10, 15 cm of stubble height each and subjected to three harvest frequencies as 2, 4, and 6 weeks. Overall results indicated that (i) there was an interaction period evaluation × treatment (P<0.05) for N2O flux and ammonia (NH3) volatilization due to contrasting gases emission initially but not at later measurements from livestock dung. Dung beetle species affected N2O flux on dung, increasing the fluxes in the 6th day (80 g N2O-N ha-1 day-1) compared to treatment with just soil and dung (2. 6 g N2O-N ha-1 day-1). D. gazella as isolated species removed and buried more dung than other isolated species and other combining species assemblages. Dung application have affected the HA of pear millet regardless the presence of dung beetle, compared with treatments with just soil with average of 8 g DM terraria-1 and 5 g DM terraria-1, respectively. A PCA analysis was used to understand the variation and correlation of each variable, which showed a low principal component explanation (less than 80%) not enough to explain the variation of the results; (ii) there was genotype × evaluation interaction effects (P<0.05) for HA, CP, and Ny. The 10 bermudagrass genotypes showed significant variation for all characteristics, being 286 more productive (P<0.05) than Jiggs in the first evaluation with 4427 kg DM ha-1 and 3245 kg DM ha-1, respectively and 323 had greater (P<0.05) CP in the fifth evaluation than 283, 286, 322 and Tifton 85 with average CP of 155, 128, 136 and 137 g kg-1 of DM, respectively.; (iii) there was effect of the treatments (P<0.05) on %Ndfa, HA, THA, Ny, TNy, BNF and CP. The genotype UF2015-AP produced 3525 kg DM ha-1 harvest-1, which is greater (P<0.05) than Bulldog 805 harvested every 6-wk. Average LC, CP, and IVDOM were 36%, 175 g kg-1 of DM, and 540 g kg-1 of DM in the first evaluation, and these values were greater (P<0.05) than the ones observed in evaluation three. Overall results indicated that dung beetles are beneficial for N cycling increasing plant productivity. There are bermudagrass genotypes that are more efficient in N utilization; and finally, harvesting management affect productivity responses of alfalfabermudagrass mixtures, but overall, this grass-legume combination did not persist for more than one year in North Florida.