Heinrich W. Scherer

Sulphur in crop production — invited paper

The fact that crop deficiencies of sulpher (S) have been reported with increasing frequency over the past several years greater attention has been focused on the importance of this element in plant nutrition. In many regions in the world S deficiency has been recognized as a limiting factor for crop production. Especially in Western Europe incidence of S deficiency has increasingly reported in Brassica over the last decade. While sulphate in soil is a direct S source for plants, generally more than 95% of soil S is organic bonded and divided into sulphate ester S and carbon-bonded S. Although not readily plant available, organic S compounds may potentially contribute to the S supply of plants via mineralization. While sulphate esters play an important role in the short term release of sulphate, carbon-bonded S seems to be responsible for long-term mineralization. Predicting the S requirement of plants has become increasingly importance in European agriculture. To predict S delivery of soils different soil testing methods are suggested, which take the most important soil characteristics into consideration. It is suggested to elaborate a sulphur test in field experiments with the most sensitive crop in rotation, like oilseed rape. Besides soil test tissue analysis assist in identifying S deficiencies. However, in general they are used very seldom for making fertilizer S recommendations. To achieve high yields and to minimize S leaching, rates of fertilizer S should be recommended on the basis of available soil S and crop requirement.

N2 fixation and growth of legumes as affected by sulphur fertilization

The influence of three sulphur application rates in combination with two nitrogen application rates on N2 fixation and growth of different legumes was investigated. N was applied as N-labelled 15NH415NO3. The 15N isotope dilution technique was used to estimate N2 fixation. At both N increments dry matter yield was highest with high S supply. Independently of the N supply, the high S application rate resulted in a significantly higher N accumulation, which was mainly caused by a higher N2 fixation rate. With the grain legumes the weight of nodules was increased by the high S application rate. The higher number of nodules per pot with optimum S supply was the result of a better root growth. Rates of acetylene reduction correlated significantly with S supply.

Dynamics and availability of the non-exchangeable NH4-N — a review

Abstract Fixation of ammonium results from the entrapment of NH4 ions in the interlayers of 2 :1 clay minerals. Many mineral soils are known to fix variable amounts of ammonium depending on the clay content, the kind of clay minerals and the degree of K saturation. After the penetration of the NH4 ions into the interlayers they are no longer prone to nitrification, resulting in a reduced leaching of nitrate into the groundwater or lower N losses by denitrification. Recent investigations suggest that the recently fixed NH44+ may be slowly released during the growing season and thus contribute to the N nutrition of the plants. For this reason non-exchangeable NH4+ should be considered as a potential available N source when modeling soil N availability and interpreting data from N fertilizer experiments. From the agronomic point of view, the non-exchangeable NH4-N should be taken into consideration in N fertilizer recommendations.

Fixation and release of ammonium in flooded rice soils as affected by redox potential

Abstract In pot experiments with two typical paddy soils, we studied the significance of ammonium fixation under waterlogged conditions and the availability of this N fraction for wetland rice. Special attention was given to the influence of redox potential ( E h ) on the fixation and release of NH + 4 , since the E h has been shown to affect the charge conditions of certain expandable clay minerals and may alter their cation exchange capacity (CEC). The results demonstrate that ammonium formed by mineralization after flooding was converted, to a substantial degree, into a non-exchangeable form when sufficient amounts of expandable 2:1 minerals were present. The newly fixed NH + 4 was protected from N losses via nitrificationn–denitrification processes, which may occur, especially during the drying and rewetting of the soil, but was completely available to the following rice crop. The release of fixed NH + 4 was highest in the rhizosphere of rice plants (where the E h was greatly increased by the O 2 secretion of the roots) and decreased with growing distance from the roots. Correlations between the E h and the concentration of non-exchangeable NH + 4 indicate that fixation and mobilization of ammonium are strongly influenced by the redox potential in paddy soils.

Arylsufatase activity in the rhizosphere and roots of different crop species

Abstract It is assumed that arylsulfatase contributes to the mineralization of organic S to SO42− for plant uptake. However, the impacts of agronomic measures (long-term organic manure) or crop species on enzyme activity are not well understood. Therefore, in the present investigations arylsulfatase activity was quantified within the rhizosphere of Sinapis album, Lolium perenne, Triticum aestivum and Brassica napus, grown on three different soils from a long-term field experiment. Highest activities were found within 0.25 mm from the root surface. Highest over all activities were found with B. napus and T. aestivum, lowest with S. album and L. perenne at a distance of 1.75 mm. As compared to soil supplemented with mineral fertilizer, compost and farmyard manure, respectively, arylsulfatase activity was higher in the soil with long-term compost application. In the protein extracts of sterile-grown roots a low arylsulfatase activity could be detected. The enzyme is induced under sulfate deficiency conditions. Preliminary biochemical data obtained from studies with T. aestivum arylsulfatase indicate conformity with biochemical properties of arylsulfatases originated from micro-organisms, vertebrates and invertebrata. From our results we assume that arylfulfatase activity determined in roots of sterile-grown plants is derived from endophytic bacteria and not by higher plants.

Sorption-desorption behavior of atrazine on soils subjected to different organic long-term amendments.

Sorption of atrazine on soils subjected to three different organic amendments was measured using a batch equilibrium technique. A higher K(F) value (2.20 kg(-1)(mg L(-1))(-N)) was obtained for soil fertilized with compost, which had a higher organic matter (OM) content. A correlation between the K(FOC) values and the percentage of aromatic carbon in OM was observed. The highest K(FOC) value was obtained for the soil with the highest aromatic content. Higher aromatic content results in higher hydrophobicity of OM, and hydrophobic interactions play a key role in binding of atrazine. On the other hand, the soil amended with farmyard manure had a higher content of carboxylic units, which could be responsible for hydrogen bonding between atrazine and OM. Dominance of hydrogen bonds compared to hydrophobic interactions can be responsible for the lower desorption capacity observed with the farmyard manure soil. The stronger hydrogen bonding can reduce the leaching of atrazine into drinking water resources and runoff to rivers and other surface waters.

Effect of organic manure on organic phosphorus fractions in two paddy soils

We investigated the transformation of the organic P fractions from organic manure in two paddy soils (Ultisol, Entisol) and the influence of organic manure or cellulose on organic P under anaerobic conditions. The results obtained from the P fractionation experiment indicated that during the incubation labile and moderately labile organic P fractions increased in the Ultisol and decreased in the Entisol, which might be related to the difference in the organic matter content of both soils. Immediately after the application of organic manure, a large part of labile and moderately labile organic P supplied with the manure was transformed into moderately resistant organic P, possibly Ca- or Mg-inositol P were transformed into Fe-inositol P. During anaerobic incubation, the labile forms of organic P in the soils treated with organic manure were increased along with the incubation period in the first 4 weeks. The change in the moderately labile fraction was dramatic. It increased sharply in the first 2 weeks, then decreased, which was more pronounced in the soils treated with pig faeces. The moderately resistant fraction decreased during the whole incubation period. This indicated that under anaerobic conditions, the moderately resistant fraction can be transformed into labile and moderately labile organic P fractions, perhaps as Fe3+-inositol P is reduced to Fe+2-inositol P. Cellulose as an organic substrate had an increasing effect on organic P, especially when it was combined with inorganic P. Therefore, it is suggested that the application of inorganic P fertilizer combined with organic manure may be an effective way of protecting inorganic P against intensive sorption in soils.

Depletion of non-exchangeable NH4-N in the soil—root interface in relation to clay mineral composition and plant species

Abstract Experiments were carried out with 10 soils with different clay contents and different clay composition to investigate the dynamics of non-exchangeable NH + 4 and 1 M HCl extractable NH + 4 , respectively, in the vicinity of the roots. The experimental plant was rape ( Brassica napus ). In addition the influence of plant species on the mobilization of non-exchangeable NH + 4 was investigated with ryegrass ( Lolium multiforum ) and red clover ( Trifolium pratense ) on a soil with a higher proportion of smectites and vermiculite. The depletion zone of non-exchangeable NH + 4 as well as 1 M HCl extractable NH + 4 extended to about 3 mm from the root zone in soils with a higher proportion of smectites and vermiculite, while in soils with mainly illites the depletion of both N fractions in the root vicinity was negligible. The depletion was less pronounced for red clover. It is suggested that the release of ammonium from the interlayers depends on clay mineral characteristics as well as on plant species.

Effects of pretreatment of soil samples on N mineralization in incubation experiments

SummaryWe studied the effects of pretreating soil samples (field-fresh, drying at 40° and 105°C, freezing/thawing) on N mineralization in an incubation experiment and on the dynamics of the organic N fraction extracted by K2SO4 solution. The soil samples were collected from plots in a long-term field experiment with the application of mineral fertilizer and farmyard manure. Compared with the field-fresh soil samples, freezing/thawing resulted in higher NO3−-N contents while the NH4+-N and the organic N content were increased by drying at 105°C. During the incubation period N mineralization was highest after the samples were dried at 105°C and a little lower in those dried at 40°C. After freezing/thawing the order of magnitude of N mineralization remained the same. The difference in organic N between the beginning and the end of the incubation experiment and the mineral N content at the end of the experiment were correlated significantly. Despite this correlation, however, the change in the organic N content underestimated the N mineralization rates.

Fixation and release of ammonium by clay minerals after slurry application

Abstract In a pot experiment with 16 topsoils and 7 subsoils the fixation and release of ammonium from clay minerals after slurry application was investigated. The following results were obtained: In the topsoils the fixation rate of NH4-N ranged between 10 and 100 per cent after the first and between 7 and 100 per cent after the second slurry application, while in the subsoils between 70 and 100 per cent and between 49 and 100 per cent, respectively, of the ammonium supplied with slurry were fixed in the interlayers of the clay minerals. The fixation rate was not influenced by the fertilizer history of the soils. During the growing period of the first crop (spring wheat) non-exchangeable NH4-N was mobilized and reached almost the original level in the planted soils, while in the non-planted soil, non-exchangeable NH4-N was accumulated. Incorporation of slurry before a following crop (corn, winter wheat, ryegrass) resulted in an increase in the content of non-exchangeable NH4-N, while topdressing had no influence on the content of this N fraction. During the growing period of these crops the content of non-exchangeable NH4-N decreased again in the planted soils and reached almost the original content when ryegrass was harvested, while in the non-planted soils non-exchangeable NH4-N was accumulated.