Interactive effects of straw mulch, soil texture, and precipitation on biomass and species diversity in a semi-desert grassland

Background/Question/Methods: Straw mulch cover has been shown to have multiple beneficial effects on soil properties including mitigation of surface temperature variability, preservation of soil moisture, increase of organic matter input, and creation of microtopographical heterogeneity. These potential benefits are desirable for revegetation efforts in water-limited environments such as a semi-desert grassland near Tucson, Arizona USA. Mulching is used as a seedbed preparation method to enhance establishment success and productivity. However, few studies have been done to examine the effects of mulching on the richness and diversity of a revegetation plant mix, nor its interactions with soil texture and precipitation. The information can be useful for determining whether to mulch in rangelands with coarse texture soils and for evaluating the cost-effectiveness of such treatment given alternative precipitation scenarios. A greenhouse experiment with four treatment variables (i.e., soil texture, amendment, irrigation amount, and seed mix) was conducted to address these questions. The three soil textures were: sand, sandy loam, and loamy sand. The amendment treatment levels were: control; 4.5 Mg/ha of straw mulch sprayed with an acrylic copolymer on top (straw); and, 4.5 Mg/ha of straw mulch and acrylic copolymer plus 3.4 Mg/ha of 7-2-3 Biosol fertilizer (straw + fertilizer). The irrigation levels were adjusted to simulate low, average, and high annual precipitation (245 mm, 384 mm, and 510 mm). The four seed mixes composed of 10 to 11 annual and perennial herbaceous and woody native plant species with similar composition by functional groups.

Results/Conclusions: The results showed significant interactive effects between amendment and soil texture and between amendment and irrigation amount (p<0.001). Straw addition increased aboveground biomass (1.0 Mg/ha and 1.2 Mg/ha) and Shannon’s diversity index (1.22 and 1.20) for both the sandy loam and loamy sand soils, but not for the sandy soil (0.3 Mg/ha and 0.69). Straw also narrowed the difference in these metrics given average or high simulated rainfall, but not the low rainfall treatment (1.0 Mg/kg and 1.1 Mg/kg vs. 0.5 Mg/ha; 1.08 and 1.18 vs. 0.86). Adding straw + fertilizer did not improve these metrics for the same combination of soil texture or irrigation level.  These results have management implications by demonstrating that mulching may not be worthwhile in drier years and on sandy soils.