In an Ecuadorian pasture soil the growth of Setaria sphacelata, but not of soil microorganisms, is co-limited by N and P

In the mountain rainforest region of southern Ecuador, soils of active pastures, established after slash- and-burn of the forest, are characterized by improved quantity and quality of soil organic matter favoring microbial conditions. However, these beneficial conditions decrease with increasing pasture age and burning frequency. As a consequence, rates of soil nutrient cycling decrease, supporting the infestation of bracken fern and, in turn, causing further decreases in pasture productivity. Finally, farmers are forced to abandon the degraded pastures and to establish new ones by continuous deforestation. To investigate whether an application of N and/or P nutrients to an extensively grazed pasture (active pasture) can improve grass productivity and maintain soil fertility, a pasture fertilization experiment was conducted. On an active pasture site, planted with Setaria sphacelata, moderate rates of urea (50 kg N ha(-1) a(-1)), rock phosphate (10 kg P ha(-1) a(-1)), and a combination of both were applied. It was examined whether soil mineralization (gross and net N mineralization, SOC mineralization) and microbial community structure (PLFA-analysis), as well as quantity and quality of the grass biomass, were affected by fertilization. Furthermore, the impact of fertilization on in situ soil respiration rates was studied. The combined application of urea and rock phosphate increased the pasture yield by 2 Mg ha(-1) a(-1) most efficiently, indicating a co-limitation of growth. Additionally, the fodder quality was improved by a 67% higher content of P and by a 7% higher content of Ca in the grass biomass compared to the control. While carbon, nitrogen, and phosphorus in the microbial biomass remained unaffected and the microbial activity increased only temporarily, the relative abundance of fungi (18:2n6,9) increased significantly due to fertilizer addition. Urea addition induced a short-lived increase in the in situ soil CO2-C effluxes, assuming a positive priming effect due to an activation of soil microbes. In total, plots amended with urea emitted 0.8 Mg CO2-C ha-(1) a(-1) more than the control. Results reveal that already moderate fertilization significantly improved pasture productivity and maintained soil quality. However, it is expected that higher loads of NP fertilizer will increase pasture productivity at the expense of soil organic carbon sequestration due to enhanced soil CO2-C losses. Hence, to establish a sustainable pasture management in the study region, the soil C management must also be carefully considered. (C) 2012 Elsevier B.V. All rights reserved.