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Individual study: Effects of organic amendments on soil biota of a degraded native grass rangeland on the Rio Puerto watershed, New Mexico, USA

Published source details

Whitford W.G., Aldon E.F., Freckman D.W., Steinberger Y. & Parker L.W. (1989) Effects of organic amendments on soil biota on a degraded rangeland. Journal of Range Management, 42, 56-60


Restoration of degraded rangelands via regeneration of native grasses may require measures to rehabilitate soil, including soil biota. This study examined soil microflora and fauna changes resulting from low levels of organic material additions to a degraded, blue grama Bouteloua gracilis,semiarid native grassland on the Rio Puerto watershed, New Mexico (southwest USA). The watershed has a history of heavy livestock grazing and degradation dating back over 200 years.

Four blocks were established, each containing four 30 x 40 m plots. In September 1980, mulch treatments (applied to surface covering 60-70% of surface area) were randomly assigned to plots by block:
1) no mulch (control);
2) wheat straw at 2 Mg/ha;
3) bark and wood chip at 2 Mg/ha;
4) dried municipal sewage sludge at 1 Mg/ha.
Plots were sampled (soil cores 6 cm diameter x 15 cm deep; 3 replicate core pairs/plot) for microarthropods and nematodes in mid-September (5 days prior to mulching) and late September (5 days after); in 1981 at 30-40 day intervals; and in 1982 in winter, early dry season (May), and twice in summer (wet growing season). A subsample was used for gravimetric water and organic matter content, and soil and litter respiration analyses. Every third month, cores were taken for microflora and protozoa biomass estimation.
Aboveground vegetation was harvested (within 12, 0.25 m² quadrats/plot) each September to obtain annual biomass estimates.
Decomposition rates were determined using straw-filled mesh litter bags placed on the soil surface (5 bags/treatment in year 1; left for 3, 6 and 9 months), or buried at 5 cm deep for 12 months (40 bags/plot in 1 block in year 2).

The main result was that soil biota in response to mulching were mostly apparent only in the first year (primarily increased nematode and microarthropod densities in bark and wood chip plots).
In the first year, straw in surface-placed bags lost significantly more weight on control, bark and wood chip, and sludge plots, than on straw mulched plots (percent losses: no mulch -18.2 %; bark and wood chip - 17.7 %; sludge - 21.2 %; straw - 11.0 %).
In the second year, perhaps due to below average rainfall, treatments had little or no effect on decomposition, litter or soil respiration, soil micro-flora biomass, or populations of most soil biota. Sludge application (albeit at low rate) produced no measured soil benefits. Soil organic matter content was similar across all treatments (average for all soils 3.8 %).
Bark and wood chips were still visible after two years (perhaps providing physical soil benefits as well as chemical enhancement), straw and sludge were not.
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