Marta Benito

Dynamics of pruning waste and spent horse litter co-composting as determined by chemical parameters

Co-composting of pruning waste and horse manure was monitored by different parameters. A windrow composting pile, having the dimensions 2.5m (height) x 30m (length) was established. The maturation of pruning waste and horse manure compost was accompanied by a decline in NH(4)(+)-N concentration, water soluble C and an increase in NO(3)(-)-N content. Organic matter (OM) content during composting followed a first-order kinetic equation. This result was in agreement with the microbiological activity measured by the CO(2) respiration during the process. The correlation at a high level of probability found between the OM loss and CO(2) evolution showed that both parameters could be used to indicate the degree of OM degradation that is the maturity and stability phases of the compost studied. Humification parameters data from the organic matter fractionation did not show a clear tendency during the composting time, suggesting that these parameters are not suitable for evaluating the dynamics of the process.

Soil CO2 Efflux in a Mixed Pine-Oak Forest in Valsaín (Central Spain)

Soil-surface CO2 efflux and its spatial and temporal variation were investigated in a southern Mediterranean, mixed pine-oak forest ecosystem on the northern slopes of the Sierra de Guadarrama in Spain from February 2006 to July 2006. Measurements of soil CO2 efflux, soil temperatures, and moisture were conducted in nine 1963-m2 sampling plots distributed in a gradient around the ecotone between Pinus sylvestris L. and Quercus pyrenaica Lam. forest stands. Total soil organic matter, Walkey-Black C, particulate organic matter, organic matter fraction below 53 μm, total soil nitrogen content, total soil organic carbon content, and pH were also measured under three representative mature oak, pine, and mixed pine-oak forest stands. Soil respiration showed a typical seasonal pattern with minimums in winter and summer, and maximums in spring, more pronounced in oak and oak-pine stands. Soil respiration values were highest in pine stands during winter and in oak stands during spring and summer. Soil respiration was highly correlated with soil temperatures in oak and pine-oak stands when soil moisture was above a drought threshold of 15%. Below this threshold value, soil moisture was a good predictor of soil respiration in pine stands. Greater soil organic matter, particulate organic matter, Walkey-Black C, total organic C, and total N content in pine compared to oak sites potentially contributed to the greater total soil CO2 efflux in these stands during the winter. Furthermore, opposing trends in the organic matter fraction below 53 μm and soil respiration between plots suggest that in oak stands, the C forms are less affected by possible changes in use. The effects of soil properties on soil respiration were masked by differences in soil temperature and moisture during the rest of the year. Understanding the spatial and temporal variation even within small geographic areas is essential to assess C budgets at ecosystem level accurately. Thus, this study bears important implications for the study of large-scale ecosystem dynamics, particularly in response to climatic change.

Carbon Mineralization of Pruning Wastes Compost At Different Stages of Composting

To study the carbon mineralization of pruning waste compost, four samples originated from pruning waste, leaves and grass clippings were collected each from a different pilot pile at different stage of the composting process: initial nondecomposed material (C1); two-months old at the end of the biooxidative stage (C2); seven months old during the curing phase (C3) and 12 months old at the end of the curing phase (C4). The CO2-C evolution was measured during 56 days of aerobic incubation. The proportion mineralized from the different composts (% of compost TOC) during the incubation period were: 4.54, 2.43, 1.71 and 1.60 for C1, C2, C3 y C4, respectively. Regardless of compost age, C mineralization occurred in two phases: a first rapid phase (corresponding to the decomposition of the most labile products by microorganisms) and a second, slower phase, during which the most resistant organic products mineralized. During the first stage, the model was fitted to a first-order equation, whereas in the second phase the model was a zero-order equation. Because of the similar results obtained for samples C3 and C4, we can conclude that organic matter had similar microbial stability at both stages and the composting process could be shortened by five months.

Comparison of a gas detection tubes test with the traditional alkaline trap method to evaluate compost stability

Compost stability is an important parameter of compost quality. Among tests proposed to evaluate compost stability, microbial respiration is one of the better accepted tests. Variations in rates of CO2 evolution during composting were studied in two pilot pruning waste piles using a windrow composting system. To measure the CO2 production rate, two methods were compared: the alkaline trap test and gas detection tubes. Both respiration tests indicated increasing compost stability with processing time, but CO2 evolution rates from the alkaline trap method were higher than values from the gas detection tube method. A first-order kinetic equation was used to describe CO2 evolution over time. A linear relationship (r=0.81, p<0.01) was found between the two methods. Although both methods could distinguish unstable compost from stable compost, CO2 detection tubes were easier to use and gave results in a shorter period of time.