Enzyme Activity Dynamics in Response to Climate Change: 2011 Drought-Heat Wave.

Extreme weather events such as severe droughts and heat waves may have permanent consequences on soil quality and functioning in agroecosystems.  The Southern High Plains (SHP) region of Texas, U.S., a large cotton producing area, experienced a historically extreme drought and heat wave during 2011, which severely reduced crop production.  During the period of November 2010 to August 2011, this region received only 39.6 mm of precipitation (vs. the historical average of 373 mm) and experienced the hottest summer since record keeping began in 1911.  This study evaluated several enzyme activities important in biogeochemical cycling at four sites under a loam and a sandy loam soil with a management history of monoculture (continuous cotton) or rotation (cotton and sorghum or millet).  These soils were sampled (0-10 cm) when the most extreme drought and heat conditions were experienced (July 2011), after precipitation and a reduction in Palmer drought severity index occurred (March 2012) and after one year (July 2012, loam sites only).  All EAs, except arylsulfatase, were significantly higher under at least one soil type and/or management history in July 2011 compared to March 2012.  The activities of β-glucosidase, phosphodiesterase and aspartase were higher (40-51%) in July 2011 than in March 2012 similarly in both soils and management types.  Total C was also reduced significantly from July 2011 to March 2012 in the rotation only for both soils.  As the loam was also sampled in July 2012, our comparisons revealed a continuous decrease in many of the EAs over time (July 2011>March 2012>July 2012) such as β-glucosidase, acid phosphatase, alkaline phosphatase, urease and aspartase.  The increase (avg. 36%) in arylsulfatase activity from July 2011 to March 2012 for both soil types may indicate a different origin/location of this enzyme compared to the others measured here.  In addition, EAs continued to be a fingerprint of the soil management history as all EAs evaluated were higher in the rotation compared to monoculture for both sites during the drought/heat wave. Our study provided evidence that extreme drought and high temperatures may result in adverse effects on soil quality such as depletion of soil organic matter reserves and reduced long-term agroecosystems resistance and resiliency.