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A major uncertainty in predicting long-term ecosystem C balance is whether stimulation of net primary production will be sustained in future atmospheric CO 2 scenarios. Immobilization of nutrients (N in particular) in plant biomass and soil organic matter (SOM) provides negative feedbacks to plant growth and may lead to progressive N limitation (PNL) of plant response to CO 2 enrichment. Soil microbes mediate N availability to plants by controlling litter decomposition and N transformations as well as dominating biological N fixation. CO 2 -induced changes in C inputs, plant nutrient demand and water use efficiency often have interactive and contrasting effects on microbes and microbially mediated N processes. One critical question is whether CO 2 -induced N accumulation in plant biomass and SOM will result in N limitation of microbes and subsequently cause them to obtain N from alternative sources or to alter the ecosystem N balance. We reviewed the experimental results that examined elevated CO 2 effects on microbial parameters, focusing on those published since 2000. These results in general show that increased C inputs dominate the CO 2 impact on microbes, microbial activities and their subsequent controls over ecosystem N dynamics, potentially enhancing microbial N acquisition and ecosystem N retention. We reason that microbial mediation of N availability for plants under future CO 2 scenarios will strongly depend on the initial ecosystem N status, and the nature and magnitude of external N inputs. Consequently, microbial processes that exert critical controls over long-term N availability for plants would be ecosystem-specific. The challenge remains to quantify CO 2 -induced changes in these processes, and to extrapolate the results from short-term studies with step-up CO 2 increases to native ecosystems that are already experiencing gradual changes in the CO 2 concentration.

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A critical step in the quantification of soil quality (SQ) is the selection of SQ benchmarks. The benchmarks used in this study were SQ ratings made by 32 farmer collaborators representing a range of farming systems, scales of operation and geographic locations in the Mid-Atlantic region of USA. Soils from 45 pairs of sites identified by their farmers as having good and poor SQ were sampled over three seasons and analyzed for 19 soil parameters. Farmer judgments of SQ were based on many factors, most commonly soil organic matter, crop performance, soil water availability and erosion history. Selected individual soil parameters were normalized and integrated into an additive SQ index (SQI). Three additional indices were developed using discriminant analysis. The level of agreement between individual parameters, SQIs and farmer SQ ratings was evaluated using paired t-tests and mean percent difference values. The additive SQI was found to have the highest level of agreement with farmer SQ ratings (P<0.0001), demonstrating that a linear combination of soil parameters can be assembled that is more in agreement with holistic SQ criteria, such as farmer SQ ratings, than individual soil parameters. Extractable C from microwave (MW) sterilized soil (a measure of microbial biomass) was the individual parameter that best agreed with farmer SQ ratings (P<0.0001). Five additional soil C parameters, as well as aggregate stability, also agreed well with farmer SQ ratings (all P values <0.0005). The three parameters with the highest ratio of mean percent difference to coefficient of variation (an indication of parameter reliability) were extractable C from MW sterilized soil, anthrone reactive C and macroaggregate stability (14.2, 7.7 and 3.7, respectively). Mineral fertility parameters (pH, Ca, Ca:Mg ratio, P and K) were not significantly related to farmer SQ ratings (P values >0.05). The strong relationships observed between soil C parameters, soil structural parameters and farmer SQ ratings suggest that efforts to improve SQ in the study region should focus on monitoring and enhancement of soil C and soil structure.

Field trials were conducted from 2010 to 2013 at four locations in Illinois to evaluate the impact of cover crops (cereal rye [Secale cereal], brown mustard [Brassica juncea], winter canola [B. napus], and winter rapeseed [B. napus]) on soybean [Glycine max] stands and yield, diseases, pathogen populations, and soil microbial communities. Cover crops were established in the fall each year and terminated the following spring either by using an herbicide (no-till farms), by incorporation (organic farm), or by an herbicide followed by incorporation (research farm). Although shifts in soilborne pathogen populations and microbial community structure were not detected, cover crops were found to induce general soil suppressiveness in some circumstances. Cereal rye and rapeseed improved soybean stands in plots inoculated with Rhizoctonia solani and decreased levels of soybean cyst nematode in the soil. Cereal rye increased soil suppressiveness to R. solani and Fusarium virguliforme, as measured in greenhouse bioassays. Cereal rye significantly improved yield when Rhizoctonia root rot was a problem.

We evaluated a permanganate-oxidizable carbon (C) method using whole soil and physical fractions of soil from two field experiments and found that the method is a sensitive indicator of tillage and organic input effects on soil C. However, the recommended concentration (20 mM) and volume (20 ml) of permanganate reagent do not combine to provide a sufficient excess of permanganate to maintain linearity over the range of C levels commonly found in agricultural soils. This nonlinearity may be inconsequential when permanganate-oxidizable C is used as a field indicator of soil quality, but correction is desirable for research applications. We propose a correction method that provides reasonable correction for soils within a 3× range of permanganate-oxidizable C levels and improves sensitivity to changes in soil C resulting from differences in tillage and organic-matter additions.

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