Prospects for soil carbon storage on recently retired marginal farmland.

Soil organic carbon (SOC) is strongly affected by farm cropping, which covers >10% of the earth's surface. Land retirement of marginal fields, now a global initiative, can increase SOC storage but reported accumulation rates are variable. Here, we quantify SOC in crop fields and retired marginal land in an intensely farmed 10,000 km 2 region of central North America, testing nutrients, soil texture and management as drivers of SOC storage. Overwhelmingly, SOC was associated with farm management with among-farm differences varying >fourfold (17.4-81 t ha -1) in the top 15 cm. Total farm SOC averaged 502.2 t farm -1 but again ranged widely (216-1611 t farm -1). Farm-specific SOC was often, but not always, higher on farms with N-rich silt-clay soils, and lower on sandy soils with higher P relating to former tobacco production. In contrast, within-farm SOC between crop fields and retired land did not significantly differ with time. Low SOC on retired lands was associated with persistently high soil N and P and elevated microbial respiration. Retired soils did possess substantially larger pools of lignin-rich root biomass to depths of 60 cm, which may signify eventual SOC accumulation possibly as nutrient legacies diminish. Our work shows that management legacy, interacting with soil texture and nutrients, predicts SOC more than short-term retirement. Indeed, crop fields averaged 67% of farm SOC because they represented up to 94% of total farm area - SOC retention on cropland remains a management priority, above and beyond gains with retirement. Interestingly, the largest per-volume SOC levels were in remnant forest that contained 25% of farm SOC despite only averaging 11% of farm area. Maintaining SOC stocks in farm landscapes may be more quickly attained by protecting remnant forest, with retired lands needing time to re-build SOC stocks.

Microbial processing of plant remains is co-limited by multiple nutrients in global grasslands.

Microbial processing of aggregate-unprotected organic matter inputs is key for soil fertility, long-term ecosystem carbon and nutrient sequestration and sustainable agriculture. We investigated the effects of adding multiple nutrients (nitrogen, phosphorus and potassium plus nine essential macro- and micro-nutrients) on decomposition and biochemical transformation of standard plant materials buried in 21 grasslands from four continents. Addition of multiple nutrients weakly but consistently increased decomposition and biochemical transformation of plant remains during the peak-season, concurrent with changes in microbial exoenzymatic activity. Higher mean annual precipitation and lower mean annual temperature were the main climatic drivers of higher decomposition rates, while biochemical transformation of plant remains was negatively related to temperature of the wettest quarter. Nutrients enhanced decomposition most at cool, high rainfall sites, indicating that in a warmer and drier future fertilized grassland soils will have an even more limited potential for microbial processing of plant remains.

Homogenization of freshwater lakes: Recent compositional shifts in fish communities are explained by gamefish movement and not climate change.

Globally, lake fish communities are being subjected to a range of scale-dependent anthropogenic pressures, from climate change to eutrophication, and from overexploitation to species introductions. As a consequence, the composition of these communities is being reshuffled, in most cases leading to a surge in taxonomic similarity at the regional scale termed homogenization. The drivers of homogenization remain unclear, which may be a reflection of interactions between various environmental changes. In this study, we investigate two potential drivers of the recent changes in the composition of freshwater fish communities: recreational fishing and climate change. Our results, derived from 524 lakes of Ontario, Canada sampled in two periods (1965-1982 and 2008-2012), demonstrate that the main contributors to homogenization are the dispersal of gamefish species, most of which are large predators. Alternative explanations relating to lake habitat (e.g., area, phosphorus) or variations in climate have limited explanatory power. Our analysis suggests that human-assisted migration is the primary driver of the observed compositional shifts, homogenizing freshwater fish community among Ontario lakes and generating food webs dominated by gamefish species.

Invasion and drought alter phenological sensitivity and synergistically lower ecosystem production.

Climate change and shifting species composition have influenced ecosystem-scale phenology worldwide. For instance, invasive plant species have greater vegetation phenological sensitivity to climate change than native plant species in some regions, and hence invasion could modify how ecosystem carbon gain responds to increased drought frequencies expected with climate change. Results from a 4-yr drought experiment show that invasion reduced ecosystem potential for carbon gain via increased sensitivity to reduced rainfall. Using canopy greenness (Normalized Difference Vegetation Index, NDVI) as a proxy for potential ecosystem carbon gain, we show that areas invaded by herbaceous species had up to a 70% reduction in maximum NDVI under severe drought conditions as compared to areas dominated by native shrubs. Phenological differences between herbaceous- and shrub-dominated vegetation contributed to this reduction in potential ecosystem carbon gain because invaded areas had delayed green-up, especially under drought conditions, and shrub senescence was accelerated by drought. Hence, invasion by herbaceous species and increased drought frequencies are likely to act synergistically to reduce ecosystem capacity for carbon gain in this system. Our findings suggest that predicting ecosystem responses to future climate change could be improved by projecting of the spread of invasive species and accounting for phenological variation between native and invading species.