Soil modulates the effect of precipitation seasonality on bioenergy crop production

Background/Question/Methods

Future climate change scenarios remain uncertain with respect to precipitation amounts and variability. In the U.S. Great Plains, spring precipitation is expected to decrease in the lower Great Plains but increase 20%–40% in the upper Mississippi Valley, suggesting potential changes in regional patterns of ecosystem function and productivity. Precipitation limits primary production by affecting soil moisture, and soil type interacts with soil moisture to determine soil water availability to plants.  We used ALMANAC, a process-based model, to simulate switchgrass (Panicum virgatum var. Alamo) biomass production in Central Texas under three different precipitation scenarios and two nitrogen inputs (100 or 200 kg N/ha) on two soil types (silty-clay and clay soils). Our objectives were to assess responses in biomass production and temporal stability to shifts in precipitation seasonality, and to determine the role of soil texture and nitrogen limitation in this response. Precipitation scenarios were: 1) wetter spring, shifting 25% of winter precipitation to spring, 2) wetter winter, shifting 25% of spring precipitation to winter, or 3) mean precipitation seasonality, based on the site’s historical weather. Mean total precipitation amounts did not differ among scenarios. 

 Results/Conclusions

With high nitrogen inputs, silty-clay soils were 1.13 tons/ha.yr more productive than clay soils (8.5%, p=0.007). The wetter spring scenario was the most productive (p=0.027), and clay soils gained more production than silty-clay soils because plant available water dynamics. With lower nitrogen inputs, plant production was lower in silty-clay than in clay soils by 4.1 tons ha-1 yr-1 (45%), because the silty-clay soils experienced between 104-112 N-stress days per year, compared to 5-13 N-stress days per year on the clay soils, a 10-fold increase in N-stress in both soils compared to the high nitrogen scenario. Precipitation scenarios at lower nitrogen inputs had no effect on production, suggesting that nitrogen limitation may have outweighed effects of seasonality, especially on the silty-clay soils. Temporal stability in productivity was higher on silty-clay than clay soils at high N; stability was highest on the clay soils at low N, indicating that nutrient inputs stabilized productivity on the more N-limited soil. Controls of productivity and its temporal stability varied with soil type, because of differences among soils in water versus N stress. Thus, climate variability effects on switchgrass biomass production and its temporal stability will vary spatially across the landscape because of soil-texture related changes in limitation by water versus nitrogen.