Permanganate-oxidizable C (POXC) and mineralizable C (as determined by short-term aerobic incubation of rewetted soil) are measures of active organic matter that may provide early indication of soil C stabilization and mineralization processes. To date, the relationship between these two promising active organic matter tests has not been comprehensively evaluated, and little is known about their functional role in the soil ecosystem. Here, we examined the relationship between POXC and mineralizable C across a wide range of soil types, management histories, and geographic locations across the United states (13 studies, 76 total sites; n = 1071) and the ability of POXC and mineralizable C to predict crop yield and total aboveground biomass. Results from this comparative analysis showed that POXC and mineralizable C are related (r 2 = 0.15-0.80) but that the relationship was differentially influenced by management practices. Overall, POXC better reflected practices that promote organic matter accumulation or stabilization and therefore can be a useful indicator of long-term soil C sequestration. Conversely, mineralizable C better reflected practices that promote organic matter mineralization and therefore can be a useful indicator of short-term soil nutrient availability. Our results also show that both mineralizable C and POXC were better predictors of corn (Zea mays L.) grain yield, aboveground biomass, and tomato (solanum lycopersicum L.) fruit yield than other soil C fractions evaluated here. Thus, the integrated use of POXC and mineralizable C can provide a complementary framework to assess the relative dynamics of soil C stabilization and nutrient mineralization functions in agroecosystems. O f the three pools that constitute soil organic matter (SOM), the active or labile pool is comprised of rapidly cycling organic material that mostly turns over in a shorter time frame (days to a few years) relative to the intermediate (a few years to decades) and stable (decades to centuries) pools (Cambardella and Elliott, 1992;Gregorich et al., 1994;Parton et al., 1987;Wander, 2004 Core Ideas• POXC and mineralizable C were evaluated across diverse agroecosystems.• The two are related but differentially influenced by management practices.• POXC better reflected sOM stabilizing practices.• Mineralizable C reflected sOM mineralizing practices.• Both predicted agronomic performance better than other soil C fractions.

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Following the decline of industrial manufacturing, many US cities have experienced severe population reductions that have resulted in large areas of vacant land. Urban agriculture has emerged as a desirable land use for these spaces, but degraded soils are common. Therefore, we measured soil and plant responses to amendments and management in urban lots where vacant houses had recently been demolished in Youngstown, OH, USA. Soil degradation was observed following demolition activities in the form of compaction (bulk density of 1·5-1·8 Mg m À3 ) and low soil microbial biomass C (21 mg C kg À1 soil). Our split-plot experiment measured the effects of organic matter (OM) amendments produced from yard wastes and the use of raised beds on soil properties and vegetable crop yields. Two years after their application, OM amendments resulted in significant improvement to a number of soil physical, chemical, and biological properties. Vegetable crop yields were improved by OM amendments in 2011 and by both OM amendments and the use of raised beds in 2012. A soil quality index, developed using factor analysis and the Soil Management Assessment Framework, produced values ranging from 0·60 to 0·85, which are comparable to those reported for rural agricultural soils. All results indicate that urban agriculture can be productive in vacant urban land and that amendments produced from urban yard wastes can improve soil quality at previously degraded sites and increase crop yields for urban agriculture.

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Core Ideas Infrared spectroscopy and chemometrics can be used to quantify soil labile C. Soils amended with organic materials need to be taken into account during calibration development. Selection of spectral regions can give parsimonious predictions of soil total and labile C. Labile soil carbon is an important component of soil organic matter because it embodies the mineralizable material that is associated with short‐term fertility. Permanganate‐oxidizable C (POXC) is a widely used method for the study of labile C dynamics in soils. Rapid methods are needed to measure labile C, and better understand how this pool varies with soil C at regional scales. Infrared spectroscopy is an inexpensive way to quantify SOC and observe fluctuations in C functional groups. Using a sample set that encompassed several soil types and plant communities (seven different research projects, n = 496), soils were analyzed via diffuse reflectance Fourier transformed mid‐infrared (MidIR, 4000–400 cm–1) and near‐infrared (NIR, 10000–4000 cm–1) spectroscopy. Spectral data were used to develop calibrations for POXC, soil organic C (SOC), and total N (TN) using partial least squares (PLS) regression. The MidIR predicted POXC slightly better than the NIR, with calibration and/or validation R2 values ranging from 0.77 to 0.81 depending on spectral pretreatments. Predictions for POXC were better than SOC and TN, but site variability influenced the calibration quality for SOC and TN. Using a selected MidIR region, which included bands correlated to POXC (3225–2270 cm–1), reduced the calibration quality, but still gave acceptable R2 values of 0.76 to 0.77 for the calibration and validation sets. We show that POXC can be predicted using NIR and MidIR spectra. Selecting informative spectral bands offers an alternative to using full spectra for PLS regressions.

There have been many studies on the effects of tillage on erosional losses from soil, but rarely have soil organic carbon (SOC), nitrogen (N) and phosphorus (P) losses been quantified simultaneously during a single erosion event. We applied a simulated rainfall event (70 mm hour −1 ) to plots within a gently sloping field (6%) in Ohio, USA, on which maize (C4) cultivation had replaced C3 vegetation several decades earlier. The plots were under different tillage management: (i) no till (NT100) for 42 years; (ii) NT100 plots from which 50% (NT50) or (iii) 100% (NT0) of crop residues were removed annually for 8 years; (iv) NT100 plots tilled 24 hours previously (TNT); and (v) conventional tillage (CT) for 28 years. Relationships between SOC, N and P concentrations and natural abundance 13 C : 15 N stable isotope values in the topsoils and sediments suggested that eroded SOC and TN were associated with the erosion of soil organic matter, whilst P losses were driven by the transport of the mineral fraction. Stable 13 C isotope analyses revealed that tillage and residue removal both increased the proportion of older (C3), rather than new (C4, maize-derived), SOC in eroded sediments. This study therefore demonstrated that a single tillage event after 42 years of continuous no-till caused larger erosional fluxes than 8 years of continuous removal of all maize residues, and that long-term conventional tillage resulted in the loss of a greater amount of older (> 28 years) SOC in eroded sediments, compared with continuous NT management.

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