Jesper Luxhøi

Alternate partial root-zone irrigation induced dry/wet cycles of soils stimulate N mineralization and improve N nutrition in tomatoes

Given the same amount of irrigation volume, applying alternate partial root-zone irrigation (PRI) has improved crop N nutrition as compared to deficit irrigation (DI), yet the mechanisms underlying this effect remain unknown. Therefore, the objective of this study was to investigate whether PRI induced soil dry/wet cycles facilitate soil organic N mineralization hereby contributing to the improvement of N nutrition in tomatoes. The plants were grown in split-root pots in a climate-controlled glasshouse and were subjected to PRI and DI treatments during early fruiting stage. 15N-labeled maize residues were incorporated into the soils. Results showed that PRI resulted in 25% higher net 15N mineralization than did DI, indicating that the enhanced mineralization of soil organic N alone could account for the 16% increase of N accumulation in the PRI than in the DI plants. The higher net N mineralization under PRI was coincided with an intensified soil microbial activity. In addition, even though soil chloroform fumigation labile carbon (CFL-C, as an index of microbial biomass) was similar for the two irrigation treatments, a significant increase of chloroform fumigation labile nitrogen (CFL-N) was found in the PRI wetting soil. Consequently, the C:N ratio of the chloroform fumigation labile pool was remarkably modified by the PRI treatment, which might indicate physiological changes of soil microbes or changes in labiality of soil organic C and N due to the dry/wet cycles of soils, altering conditions for net N mineralization. Moreover, in both soil compartments PRI caused significantly less extractable organic carbon (EOC) as compared with DI; whilst in the PRI wetting soil significantly higher extractable organic nitrogen (EON) was observed. A low EOC:EON ratio in the PRI wetting soil may indicate an increasing net mineralization of the organic N as a result of microbial metabolism. Conclusively, PRI induced greater microbial activity and higher microbial substrates availability are seemingly responsible for the enhanced net N mineralization and improved N nutrition in tomato plants.

Phosphorus distribution in untreated and composted solid fractions from slurry separation.

The distribution of phosphorus (P) (water-soluble inorganic P [P(i)], acid-soluble P(i), and residual P) was determined in 40 samples of solids from solid-liquid separated slurry. These were collected from separation plants using different technologies, separating by simple mechanical means, by flocculation as pre-treatment before mechanical separation or by anaerobic digestion followed by separation and centrifugation. Simple mechanical separation yielded a low solid TP content (8-9 g P kg(-1) dry matter [DM]) compared with separation by flocculation (26 g P kg(-1) DM) or by anaerobic digestion-centrifugation (33.4 g P kg(-1) DM). Acid-soluble P(i) predominated in the high P-yielding solids, whereas organic-bound or residual P was a minor component in all slurry solids. Acid-soluble P(i) and residual P were significantly correlated with total phosphorus (TP) content (R(2) = 0.855 and R(2) = 0.584), but water-soluble P(i) was uncorrelated (R(2) = 0.077). The relative distribution of P(i) to TP in the solids showed a high proportion of water-soluble P(i) in solids from simple mechanical separation, whereas the absolute concentrations were highest in solids from separation by flocculation and anaerobic digestion-centrifugation. Three solid fractions, representing the range of solids variability produced by the separation techniques, were composted for 30 d, and the P distribution was compared before and after composting. Total mass of P was conserved during composting, but water-soluble P(i) as a proportion of TP decreased in most cases. The most pronounced decrease in water-soluble P(i) was observed during composting of the solids separated using flocculation. However, changes in short- to medium-term bioavailability of P were modest, and thus the potential benefits of composting regarding storage and handling can presumably be realized without seriously compromising the P fertilization quality.

Gross N mineralization–immobilization rates in heterogeneous intact soil cores can be estimated without marked error

Gross rates of N mineralization–immobilization turnover (MIT) and gross nitrification in soil can be determined by use of a 15NH4+ pool dilution technique, under the assumption that native 14NH4+, applied 14NH4+ and microbial processes are uniformly distributed in the soil. In a laboratory investigation we compared gross N fluxes obtained from two labelling techniques applied to an arable sandy loam: (1) injection of 15NH4+ solution into intact soil cores, and (2) mixing 15NH4+ solution into disturbed soil. It was assumed that MIT obtained with the mixing technique reflected the “true” rates, since the assumption of uniformity was thought to be satisfied by this technique. MIT from the two techniques were not significantly different, thus non-uniform distribution of native 14NH4+, injected 15NH4+ and microbial processes in the intact-core technique did not cause a marked error in the MIT rates. In contrast the gross nitrification rates were twofold higher with the mixing technique than with the intact-core technique. Gross nitrification rates are likely to increase with the mixing technique because mixing redistributes nitrifiers and added 15NH4+, and thereby increases the contact between NH4+ and nitrifiers.

Carbon and N turnover in moist sandy soil following short exposure to a range of high soil temperature regimes

Laboratory experiments were undertaken to examine the effects of high soil temperatures on N biotransformations in sandy soils. Soils were incubated at 308 ,4 08 ,5 08, and 608C for 2 days, before all treatments were kept at 308C for up to 41 days. Another laboratory experiment evaluated the effect of different cycles of exposure to 508 and 308C, including frequency and duration of exposure to 508C, to assess the sensitivity of N biotransformations to temporary increases in temperature in the high range. CO2-C production, soil microbial biomass-C, gross N mineralisation, gross N immobilisation, and potential gross nitrification were measured. Gross N mineralisation and CO2-C production increased with temperature (in the range 308508C) and exhibited a Q10-relationship close to 2. Between 508 and 608C, Q10 was closer to 2.8. The increase in gross N mineralisation and CO2-C production after exposure to 508 and 608C is attributed to the decomposition of dead microbial biomass by the viable microbial population but this flush in activity was short-lived. Immobilisation rate was always low and remained unaffected by the temperature regime, probably because the growth of the microbial biomass was inhibited at the higher temperatures. This imbalance between gross N mineralisation and immobilisation resulted in rapid increases in mineral N in soil. Two 6-h cycles of 508C interspersed with 308C were equally as effective as a single 48-h exposure at stimulating CO2 production. Evidence of uncoupling CO2 production and gross N mineralisation was observed in one study where soil was incubated at 508C, but this response was not universal. The nitrification process was totally suppressed by exposure to temperature higher than 408C, probably due to thermal denaturation of enzymes. The relevance of findings to field conditions is discussed.

Application rate and composting method affect the immediate and residual manure fertilizer value in a maize–rice–rice–maize cropping sequence on a degraded soil in northern Vietnam

A field experiment was carried out in northern Vietnam to investigate the effects of adding different additives [rice (Oriza sativa L.) straw only, or rice straw with added lime, superphosphate (SSP), urea or a mixture of selected microorganism species] on nitrogen (N) losses and nutrient concentrations in manure composts. The composts and fresh manure were applied to a three-crop per year sequence (maize–rice–rice) on a degraded soil (Plinthic Acrisol/Plinthaquult) to investigate the effects of manure type on crop yield, N uptake and fertilizer value. Total N losses during composting with SSP were 20% of initial total N, while with other additives they were 30–35%. With SSP as a compost additive, 65–85% of the initial ammonium-N (NH4-N) in the manure remained in the compost compared with 25% for microorganisms and 30% for lime. Nitrogen uptake efficiency (NUE) of fresh manure was lower than that of composted manure when applied to maize (Zea mays L.), but higher when applied to rice (Oriza sativa L.). The NUE of compost with SSP was generally higher than that of compost with straw only and lime. The mineral fertilizer equivalent (MFE) of manure types for maize decreased in the order: manure composted with SSP > manure composted with straw only and fresh manure > manure composted with lime. For rice, the corresponding order was: fresh manure > manure composted with SSP/microorganisms/urea > manure composted with lime/with straw alone. The MFE was higher when 5 tons manure ha−1 were applied than when 10 tons manure ha−1 were applied throughout the crop sequence. The residual effect of composted manures (determined in a fourth crop, with no manure applied) was generally 50% higher than that of fresh manure after one year of manure and compost application. Thus, addition of SSP during composting improved the field fertilizer value of composted pig manure the most.

Modelling C and N mineralization during decomposition of anaerobically digested and composted municipal solid waste

Application of municipal solid waste (MSW) to arable land can be used to close the nutrient cycle between urban and rural areas. The aim of the current study was to quantify net N mineralization and respiration from composted MSW (CMSW) and anaerobically digested MSW (ADMSW) applied to soil, and to test whether a simple relationship between net N mineralization and respiration that was developed for plant materials, was applicable for these types of MSW. In a laboratory experiment, CMSW and ADMSW were incorporated into soil and incubated at 15°C. During the 149-day experiment, net N mineralization and respiration were determined. Cumulative respiration derived from both MSW types was very steep during the first 30 days, after which it levelled off. However, calculated on the basis of applied C, the ADMSW was 10 times more degradable than the CMSW. Both MSW types caused initial net N immobilization followed by re-mineralization. A simple model based on the relationship between net N mineralization and respiration was only applicable for the MSW after significant modifications. If farmers are to recognize CMSW and ADMSW as valuable fertilizers, it is important that they can be produced with higher maturity, in order to avoid initial N immobilization.

Short Comment on: Nitrogen mineralisation in relation to previous crops and pastures ∗

Angus et al. (2006) published a paper from 2 field experiments conducted in southernNewSouthWales, where they determined net nitrogen (N) mineralisation in relation to previous crops and pastures. The comprehensive study was very interesting to read but also challenging due to the complex experimental design including different methods used to remove the pasture and different subsequent crop species. A general striking result was highlighted. During the first year after lucerne+ perennial grass pasture, the net N mineralisation was higher (0.64 kgN/ha.day) than the first year after pure lucerne pasture (–0.08 kgN/ha.day) shown in table 4, page 361. The authors expected that pastures containing the highest potential inputs from N2-fixation would provide N-rich residues with a high N mineralisation potential (Peoples et al. 1998). N-richmaterial such as lucerne is expected to have an initially fast N mineralisation controlled by the breakdown of easily decomposable components, followed by a much slower decay of stabilised residues and decomposition products. Hence, the authors had no clear explanation of the higher N mineralisation after lucerne+ perennial grass. Under different climatic conditions in the Northern Hemisphere, Hauggaard-Nielsen et al. (1998) found that net N mineralisation after incorporation of clover–grass pastures grown without any mineral fertiliser application was similar to the level after incorporation of clover pure stands supplied with 150 kgN/ha.year and even with a tendency to increase soil mineral N levels after clover–grass compared with pure clover 100 days after incorporation and throughout the experimental period (130 days after residue incorporation). In the experiment by Angus et al. (2006) it is stated that fertiliser inputs followed local recommendations, but with no specific information about fertiliser strategies when having pastures including legumes. Potentially, the pastures have received mineral N fertilisers. As soil mineral N is known to reduce N2-fixation (Hogh-Jensen and Schjoerring 1997) it can be assumed that the lucerne N2-fixation at Old Junee (Angus et al. 2006) is reduced. It is expected that the perennial grass is very efficient at extracting soil mineral N, as shown for ryegrass when grown together with clover under temperate growing conditions (Hogh-Jensen and Schjoerring 1997). Such increased interspecific competition towards soil

Bio-oil Production in the Field

Bio-oil Production in the Field In the editorial “Green Plants, Fossil Fuels, and now Biofuels” (BioScience 56: 875), David Pimentel and Tad Patzek argue that bioethanol is a nonsustainable way to produce fuel from biomass. Their main argument is that the amount of energy from fossil fuel required to produce ethanol is larger than the energy contained in the ethanol, at least with current technology. The authors recommend giving priority to energy conservation and solar energy. However, we want to call attention to a more sustainable method of biofuel production. Flash pyrolysis is a thermochemical process in which biomass is converted to bio-oil in an oxygen-free atmosphere. The energy requirement for heating is less than 15% of the heating value in the processed biomass. The flash pyrolysis process developed at the Technical University of Denmark is optimized to produce bio-oil from cereal straw directly in the field (Bech and Dam-Johansen 2006). This means that the high energy consumption for transportation of biomass to the processing plant is avoided, and approximately 55% of the energy from the processed straw ends up in the bio-oil. The remaining organic material is turned into char and distributed in the field together with most nutrients. Char is generally very resistant to microbial degradation, which means that this carbon is sequestered in the soil. Some South American soils (terra preta, or dark earth), amended thousands of years ago with large amounts of anthropogenic char, currently contain up to 9% carbon, compared with 0.5% for plain soil from nearby areas (Marris 2006). In addition, the char has a long range of positive effects on factors associated with soil quality, which can help amend some of the detrimental effect that the removal of biomass has on soil quality. Hence, by integrating flash pyrolysis of biomass with char amendment, sustainable biooil can be produced, carbon can be sequestered, and soil fertility can be improved.