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The effect of tillage on the overall greenhouse gas balance of croplands is not clear. It has been suggested that ceasing tillage increases C sequestration, but has the risk of enhancing NO emission, which could switch the greenhouse gas balance from negative to positive. We studied the short-term effect of tillage intensity on NO and CO emissions. We changed reduced tillage to conventional tillage or no tillage and performed two tillage operations in two growing seasons. All other parameters such as agricultural management, climate, and crop type at the study site, an intermediately aerated Luvisol in Belgium, were similar. Nitrous oxide and CO emissions were measured event-directed about 40 times per year from September 2006 until December 2008 using a closed chamber technique. We did not observe any significant short-term effect of tillage intensity on NO emissions during the 2 years following tillage conversion. The 2-year aggregated NO emission was not affected by the absence of tillage, 5.6 kg NO-N ha, compared to conventional tillage, 4.6 ± 0.9 kg NO-N ha, or reduced tillage, 4.7 ± 0.3 kg NO-N ha. Enhanced NO emission events in the absence of tillage, 1 year after conversion, could be explained by the combination of higher N application and wetter conditions. Conversion to conventional tillage caused a small, but significant, increase in CO emission over the same period. We conclude that in the short term, none of the tillage intensities had an effect on NO emission, and the effect on CO emission was slightly positive when tillage intensity increased. A low short-term risk of increased NO emission in the absence of tillage in well-aerated croplands is beneficial agro-environmentally, but the long-term effect should also be assessed via follow-up studies. [ABSTRACT FROM AUTHOR]

Abstract Evaluations of soil organic carbon (SOC) stocks are often based on assigning a carbon density to each one of a number of ecosystems or soil classes considered, using data from soil profiles within these categories. A better approach, in which the use of classification methods by which extrapolation of SOC data to larger areas is avoided, can only be used if enough data are available at a sufficiently small scale. Over 190 000 SOC measurements (0–24 cm) have been made in the Flemish cropland (the Northern part of Belgium) in the 1989–2000 period. These SOC data were grouped into 3-year periods and as means plus standard deviation per (part of) community (polygons). This large dataset was used to calculate SOC stocks and their evolution with time, without data extrapolation. Using a detailed soil map, larger spatial groups of polygons were created based on soil texture and spatial location. Linear regression analysis showed that in the entire study area, SOC stocks had decreased or at best had remained stable. In total, a yearly decrease of 354 kton OC yr-1 was calculated, which corresponds with a net CO2 emission of 1238 kton CO2 yr-1 . Specific regions with a high carbon sequestration potential were identified, based on SOC losses during the 1989–2000 period and the mean 1999 SOC content, compared to the average SOC content of soils in Flanders with a similar soil texture. When restoring the SOC stocks to their 1990 level, we estimated the carbon sequestration potential of the Flemish cropland soils to be some 300 kton CO2 yr-1 at best, which corresponds to a 40-year restoration period. In conclusion, we can say that in regions where agricultural production is very intense, carbon sequestration in the cropland may make only a very modest contribution to a country's effort to reduce greenhouse gas emissions. [ABSTRACT FROM AUTHOR]

The effect of straw and vinasses on the N mineralization–immobilization turnover of celery residues was investigated in a field experiment in Flanders (Belgium). The field was laid out in raised beds on a loamy sand soil. At the start of the experiment, 15N-labelled celery residues (4t dry matter (DM)ha−1) were mixed with straw (12tDMha−1) in order to immobilize the released celery-N followed by an incorporation of vinasses (4tDMha−1) after 200 days aiming to remineralize the immobilized N. Total N, mineral N and their 15N enrichments as well as microbial biomass N were determined at regular time intervals. During the first 62 days after the incorporation, straw immobilized the celery derived 15N in the microbial biomass and reduced the total celery-15N losses from the top 25cm by 38%. However, after 62 days, the N immobilization ceased due to low temperatures (<10°C) in the raised beds, and was followed by natural remineralization of immobilized celery-15N (without addition of vinasses) at a moment when the risk of nitrate leaching was still high. Hence, straw was not able to reduce the celery-N losses during the complete winter period. The addition of vinasses in spring caused no real positive priming effect, although it did increase the amount of remineralized celery-15N (7.6% of celery derived 15N) compared to the straw treatment without vinasses (1.7% of celery derived 15N) probably due to an apparent added nitrogen interaction caused by displacement reactions with the soil microbial biomass. In conclusion, in raised beds, it is not possible to reduce nitrate leaching and to achieve a synchronization between the N release from crop residues and N demand by the following crop by using straw and vinasses to manipulate the N release from crop residues. [Copyright &y& Elsevier]