Abstract:
In the mountain rainforest region of southern Ecuador, soils of active pastures, established after slashand-
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.
