Bradford Mark A.

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Bradford
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Mark A.
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Preprint

Decreased mass specific respiration under experimental warming is robust to the microbial biomass method employed

2009-05 , Bradford, Mark A. , Wallenstein, Matthew D. , Allison, Steven D. , Treseder, Kathleen K. , Frey, Serita D. , Watts, Brian W. , Davies, Christian A. , Maddox, Thomas R. , Melillo, Jerry M. , Mohan, Jacqueline E. , Reynolds, James F.

Hartley et al. question whether reduction in Rmass, under experimental warming, arises because of the biomass method. We show the method they treat as independent yields the same result. We describe why the substrate-depletion hypothesis cannot alone explain observed responses, and urge caution in the interpretation of the seasonal data.

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Article

Farm management, not soil microbial diversity, controls nutrient loss from smallholder tropical agriculture

2015-03-04 , Wood, Stephen A. , Almaraz, Maya , Bradford, Mark A. , McGuire, Krista L. , Naeem, Shahid , Neill, Christopher , Palm, Cheryl A. , Tully, Katherine L. , Zhou, Jizhong

Tropical smallholder agriculture is undergoing rapid transformation in nutrient cycling pathways as international development efforts strongly promote greater use of mineral fertilizers to increase crop yields. These changes in nutrient availability may alter the composition of microbial communities with consequences for rates of biogeochemical processes that control nutrient losses to the environment. Ecological theory suggests that altered microbial diversity will strongly influence processes performed by relatively few microbial taxa, such as denitrification and hence nitrogen losses as nitrous oxide, a powerful greenhouse gas. Whether this theory helps predict nutrient losses from agriculture depends on the relative effects of microbial community change and increased nutrient availability on ecosystem processes. We find that mineral and organic nutrient addition to smallholder farms in Kenya alters the taxonomic and functional diversity of soil microbes. However, we find that the direct effects of farm management on both denitrification and carbon mineralization are greater than indirect effects through changes in the taxonomic and functional diversity of microbial communities. Changes in functional diversity are strongly coupled to changes in specific functional genes involved in denitrification, suggesting that it is the expression, rather than abundance, of key functional genes that can serve as an indicator of ecosystem process rates. Our results thus suggest that widely used broad summary statistics of microbial diversity based on DNA may be inappropriate for linking microbial communities to ecosystem processes in certain applied settings. Our results also raise doubts about the relative control of microbial composition compared to direct effects of management on nutrient losses in applied settings such as tropical agriculture.

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Preprint

Animating the carbon cycle

2013-08 , Schmitz, Oswald J. , Raymond, Peter A. , Estes, James A. , Kurz, Werner A. , Holtgrieve, Gordon W. , Ritchie, Mark E. , Schindler, Daniel E. , Spivak, Amanda C. , Wilson, Rod W. , Bradford, Mark A. , Christensen, Villy , Deegan, Linda A. , Smetacek, Victor , Vanni, Michael J. , Wilmers, Christopher C.

Understanding the biogeochemical processes regulating carbon cycling is central to mitigating atmospheric CO2 emissions. The role of living organisms has been accounted for, but the focus has traditionally been on contributions of plants and microbes. We develop the case that fully “animating” the carbon cycle requires broader consideration of the functional role of animals in mediating biogeochemical processes and quantification of their effects on carbon storage and exchange among terrestrial and aquatic reservoirs and the atmosphere. To encourage more hypothesis-driven experimental research that quantifies animal effects we discuss the mechanisms by which animals may affect carbon exchanges and storage within and among ecosystems and the atmosphere. We illustrate how those mechanisms lead to multiplier effects whose magnitudes may rival those of more traditional carbon storage and exchange rate estimates currently used in the carbon budget. Many animal species are already directly managed. Thus improved quantitative understanding of their influence on carbon budgets may create opportunity for management and policy to identify and implement new options for mitigating CO2 release at regional scales.

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Preprint

Thermal adaptation of soil microbial respiration to elevated temperature

2008-07-22 , Bradford, Mark A. , Davies, Christian A. , Frey, Serita D. , Maddox, Thomas R. , Melillo, Jerry M. , Mohan, Jacqueline E. , Reynolds, James F. , Treseder, Kathleen K. , Wallenstein, Matthew D.

In the short-term heterotrophic soil respiration is strongly and positively related to temperature. In the long-term its response to temperature is uncertain. One reason for this is because in field experiments increases in respiration due to warming are relatively short-lived. The explanations proposed for this ephemeral response include depletion of fast-cycling, soil carbon pools and thermal adaptation of microbial respiration. Using a >15 year soil warming experiment in a mid-latitude forest, we show that the apparent ‘acclimation’ of soil respiration at the ecosystem scale results from combined effects of reductions in soil carbon pools and microbial biomass, and thermal adaptation of microbial respiration. Mass specific respiration rates were lower when seasonal temperatures were higher, suggesting that rate reductions under experimental warming likely occurred through temperature-induced changes in the microbial community. Our results imply that stimulatory effects of global temperature rise on soil respiration rates may be lower than currently predicted.