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Background Various indices are applied to evaluate the nutrient (mostly nitrogen, N) use efficiency of plants, but those indices have rarely been compared across different crops, and the co-limitation of growth by nutrients other than N is usually not considered. Aims To conceptually and quantitatively compare the indices of a plant-level, a plant-soil-level and a fieldlevel (difference) method for the assessment of N use across a set of different annual and perennial crops; and to use some plant-level indices for exploring the colimitation of growth by nutrients other than N in wheat. Method Data sets from previously published studies on wheat (grain), maize (feed), potato (starch), grassland ley (feed) and Salix (bio-energy) field-grown in Sweden were re-analyzed. Conclusions This study is first in conceptually and quantitatively comparing various popular N use indices across a wide range of annual and perennial crops; and also proposes a methodology for exploring the colimitation of growth by nutrients other than N. When comparing the plant-level and plant-soil-level methods, the indices relating crop yields to the amounts of plantinternal N were correlated, while the N-uptake indices were not. Only few of the field-level (differencemethod) indices were correlated with indices of the two other methods.

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This study investigated whether small additions to soil of primary paper-mill sludge, a wood fibre residue from paper production (fibre sludge), caused temporary N immobilization and thereby reduced the amount of inorganic nitrogen leached from agricultural land. This was achieved by measuring respiration and immobilization of N in incubation studies at 88C, with fibre sludge added at rates varying from 63 to 1000 mg C kg (1 soil. Glucose added at rates of 63 Á/250 mg C kg (1 soil was used as a reference. Respiration in soil after glucose addition followed an exponential course with the highest rates on days 2 Á/4. During this period maximum peaks of net N immobilization were measured. Even addition of only 63 mg glucose-C kg (1 soil caused significant immobilization of N in soil. Fibre sludge additions to soil caused lower respiration activities, characterized by two initial peaks followed by somewhat higher respiration rates during the remaining incubation than for glucose. It was likely that hemicellulose, which amounted to 14% of the total C, was the initial available energy source in the sludge as concentrations of water-soluble C were very low. Addition of at least 250 mg C kg (1 soil as fibre sludge was required to cause significant N immobilization in soil corresponding to 5 kg N ha (1 . Both nitrate and ammonium were immobilized. Relating maximum N immobilization data during days 2 to 10 to corresponding respiration data for glucose and fibre sludge revealed that microbes utilised similar amounts of C per unit N immobilized. On average, 175.69/74.8 mg CO 2 -C were respired to immobilize 1 mg N and the relationship between C respiration and N immobilization was linear (R 2 0/0.984). To make soil application of fibre sludge a realistic counter-measure against N leaching from agricultural soils, pre-treatment is necessary to increase the content of energy readily available to microbes.

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Aiming at improving fertilizer management by assessing fertilizer dissolution, we compared the hygroscopicity of calcium ammonium nitrate (CAN), ammonium nitrate (NS 27‐4), and nitrate compounds (laboratory grade). Dissolution of N fertilizers was also studied under simulated rain conditions. All compounds were highly hygroscopic, dissolving within 24 h at 90–99% relative humidity and 25 °C. Addition of 2 mm rain to fertilizer granules (3–4.5 mm diam.) was sufficient to dissolve 50% of the compounds. Dissolution rates by humidity or rain were not the limiting step for plant availability.