Potthast, K.; Hamer, U. & Makeschin, F. (2012): <b>In an Ecuadorian pasture soil the growth of Setaria sphacelata, but not of soil microorganisms, is co-limited by N and P</b>. <i>Applied Soil Ecology</i> <b>62</b>, 103-114.
Resource Description
Title:
In an Ecuadorian pasture soil the growth of Setaria sphacelata, but not of soil microorganisms, is co-limited by N and P
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Abstract:
In the mountain rainforest region of southern Ecuador, soils of active pastures, established after slashand-<br/>
burn of the forest, are characterized by improved quantity and quality of soil organic matter favoring<br/>
microbial conditions. However, these beneficial conditions decrease with increasing pasture age and<br/>
burning frequency. As a consequence, rates of soil nutrient cycling decrease, supporting the infestation<br/>
of bracken fern and, in turn, causing further decreases in pasture productivity. Finally, farmers are forced<br/>
to abandon the degraded pastures and to establish new ones by continuous deforestation. To investigate<br/>
whether an application of N and/or P nutrients to an extensively grazed pasture (active pasture) can<br/>
improve grass productivity and maintain soil fertility, a pasture fertilization experiment was conducted.<br/>
On an active pasture site, planted with Setaria sphacelata, moderate rates of urea (50 kg N ha−1 a−1), rock<br/>
phosphate (10 kg P ha−1 a−1), and a combination of both were applied. It was examined whether soil<br/>
mineralization (gross and net N mineralization, SOC mineralization) and microbial community structure<br/>
(PLFA-analysis), as well as quantity and quality of the grass biomass, were affected by fertilization.<br/>
Furthermore, the impact of fertilization on in situ soil respiration rates was studied. The combined application<br/>
of urea and rock phosphate increased the pasture yield by 2 Mg ha−1 a−1 most efficiently, indicating<br/>
a co-limitation of growth. Additionally, the fodder quality was improved by a 67% higher content of P<br/>
and by a 7% higher content of Ca in the grass biomass compared to the control. While carbon, nitrogen,<br/>
and phosphorus in the microbial biomass remained unaffected and the microbial activity increased<br/>
only temporarily, the relative abundance of fungi (18:2n6,9) increased significantly due to fertilizer addition.<br/>
Urea addition induced a short-lived increase in the in situ soil CO2-C effluxes, assuming a positive<br/>
priming effect due to an activation of soil microbes. In total, plots amended with urea emitted 0.8 Mg CO2-<br/>
C ha−1 a−1 more than the control. Results reveal that already moderate fertilization significantly improved<br/>
pasture productivity and maintained soil quality. However, it is expected that higher loads of NP fertilizer<br/>
will increase pasture productivity at the expense of soil organic carbon sequestration due to enhanced<br/>
soil CO2-C losses. Hence, to establish a sustainable pasture management in the study region, the soil C<br/>
management must also be carefully considered.
