Relationship between the amount of reduced nitrogen accumulated in winter wheat shoots and the activity of nitrate reductase measured in situ

Nitrate reductase activity measured in shoots of winter wheat (Triticum aestivum cv. Apollo and Soissons) was assessed as an indicator of the nitrate assimilation rate in plants grown in the field and subjected to various nitrogen nutrition treatments. In N-deficient plants, the nitrate reductase activity was lower than in well fertilized plants and was an early indicator of nitrogen deficiency. At each stage of stem elongation, there was a constant relationship between the nitrate reductase activity in the shoots and the quantity of reduced nitrogen accumulated daily in these organs. This relationship was not affected by the level of nitrogen fertilization.     

Management practices of antecedent leguminous and non-leguminous crop residues in relation to winter wheat yields, nitrogen uptake, soil nitrogen mineralization and simple nitrogen balance

Many studies have been conducted in examining the effects of N fertilizers on cereal yields and nitrogen (N) uptake, the effects of different kind of crop residues and their management practices on cereal yield, nitrogen uptake and simple N balance have not been studied extensively. We studied the effects of antecedent leguminous (white clover and field pea) and non-leguminous (perennial ryegrass and winter wheat) crop residues, each subjected to four different residue management practices (ploughed, rotary hoed, mulched and burned) on grain yield, nitrogen uptake by succeeding winter wheat crops, soil N mineralization and simple N balance. Grain yield and N uptake by the first wheat crop were significantly higher under leguminous than non-leguminous residues, following the order of white clover>pea>ryegrass>wheat. Grain yield under the mulched treatment was significantly lower than those of other management treatments due to lower plant population established. While N uptake was significantly lower under rotary hoed and mulched treatments as compared to other treatments, mulching had a positive residual effect on the grain yield of second wheat crop. Similar to grain yield, total soil N mineralization was greater under leguminous residues during the growing period of first wheat crop and was significantly correlated with C/N ratio of the residues. The calculated simple N balance showed that positive N balances occurred under white clover after one wheat crop when N inputs from only crop residue tops was considered. This also occurred even after two wheat crops when total N inputs from crop residues (tops+roots) were considered. However, with pea, the positive N balance occurred only after the first wheat crop when total N input from crop residues (tops+roots) were considered. These calculations demonstrated the important contribution of root-N to the N economy of the cropping system, which was largely ignored in most studies. The burning of residues showed no significant advantage over other residue management treatments. This was also evident from N balance calculations, which showed, in general, N balance was lower or more negative under residue-burned treatment as compared with other treatments. Overall, present results showed that it is beneficial to retain crop residues in the field, even though non-leguminous residues may cause substantial soil N immobilization initially reducing N availability to the first wheat crop, this N eventually became available to subsequent wheat crops and also increase the fertility of soils in the long-term. Thus, N inputs from crop residues are far more beneficial to the cropping system as compared to the burning of crop residues in the field or their removal from the field.     

Simulation of growth and development processes of spring wheat in response to CO2 and ozone for different sites and years in Europe using mechanistic crop simulation models

The response of crop growth and yield to CO₂ and ozone is known to depend on climatic conditions and is difficult to quantify due to the complexity of the processes involved. Two modified mechanistic crop simulation models (AFRCWHEAT2-O3 and LINTULCC), which differ in the levels of mechanistic detail, were used to simulate the effects of CO₂ (ambient, ambient ×2) and ozone (ambient, ambient ×1.5) on growth and developmental processes of spring wheat in response to climatic conditions. Simulations were analysed using data from the ESPACE-wheat project in which spring wheat cv. Minaret was grown in open-top chambers at nine sites throughout Europe and for up to 3 years at each site.

Both models closely predicted phenological development and the average measured biomass at maturity. However, intermediate growth variables such as biomass and leaf area index (LAI) at anthesis, seasonal accumulated photosynthetically active radiation intercepted by the crop (ΣIPAR), the average seasonal light use efficiency (LUE) and the light saturated rate of flag leaf photosynthesis (A_sat) were predicted differently and less accurately by the two models. The effect of CO₂ on the final biomass was underestimated by AFRCWHEAT2-O3 due to its poor simulation of the effect of CO₂ on tillering, and LAI.LINTULCC overestimated the response of biomass production to changes in CO₂ level due to an overprediction of the effect of CO₂ on LUE. The measured effect of ozone exposure on final biomass was predicted closely by the two models. The models also simulated the observed interactive effect of CO₂ and ozone on biomass production. However, the effects of ozone on LAI, ΣIPAR and A_sat were simulated differently by the models and less accurately with LINTULCC for the ozone effects on LAI and ΣIPAR. Predictions of the variation between sites and years of growth and development parameters and of their responses to CO₂ and ozone were poor for both AFRCWHEAT2-O3 and LINTULCC. It was concluded that other factors than those considered in the models such as chamber design and soil properties may have affected the growth and development of cv. Minaret. An analysis of the relationships between growth parameters calculated from the simulations supported this conclusion. In order to apply models for global change impact assessment studies, the difficulties in simulating biomass production in response to CO₂ need to be considered. We suggest that the simulation of leaf area dynamics deserves particular attention in this regard.