Effects of agriculture on wood breakdown and microbial biofilm respiration in southern Appalachian streams

Abstract

Summary1. Agriculture causes high sediment, nutrient and light input to streams, which may affect rates of ecosystem processes, such as organic matter decay. In the southern Appalachians, socioeconomic trends over the past 50 years have caused widespread abandonment of farmland with subsequent reforestation. Physical and chemical properties of streams in these reforested areas may be returning to pre‐agriculture levels thereby creating the potential for recovery of ecosystem processes.2. We examined wood breakdown and microbial activity on wood substrata in streams with different historical and current agricultural activity in their catchments. We analysed historical (1950) and recent (1998) forested land cover from large areas of the southern Appalachians and categorized streams based on percent forested land cover in these two time periods. Categories included a gradient of current agriculture from forested to heavily agricultural and reforestation from agriculture due to land abandonment. We compared microbial respiration on wood veneer substrata and breakdown of wood veneers among these land‐use categories. We also compared temperature, sediment accumulation and nitrogen and phosphorus concentrations.3. Streams with current agriculture had higher concentrations of dissolved inorganic nitrogen than forested streams. Despite reforestation from agriculture, nitrogen concentrations were also elevated in streams with agricultural histories relative to forested streams. Temperature was also higher in agricultural streams but appeared to recover from historical agriculture through reforestation and stream shading.4. Wood breakdown rates ranged from 0.0015 to 0.0076 day−1 and were similar to other studies using wood veneers to determine breakdown rate. Microbial respiration increased with incubation time in streams up to approximately 150 days, after which it remained constant. Neither wood breakdown nor microbial respiration was significantly different among land‐use categories, despite the observed physical and chemical differences in streams based on land‐use. Wood breakdown rates could be predicted by microbial respiration indicating microbial control of wood breakdown in these streams. Both breakdown and microbial respiration were negatively correlated with the amount of inorganic sediment accumulated on wood veneers.5. Higher nutrients and temperature led us to expect faster breakdown and higher microbial respiration in agricultural streams, but sediment in these streams may be limiting microbial activity and breakdown of organic material resulting in little net effect of agriculture on wood breakdown. Wood may not be desirable as a tool for functional assessment of stream integrity due to its unpredictable response to agriculture.