Mary K. Turner

Interactions between crop residue and soil organic matter quality and the functional diversity of soil microbial communities

Abstract The effect of crop residue and soil organic matter (SOM) quality on the functional characteristics of soil microbial communities was investigated. Five shoot and root materials with contrasting biochemical qualities were incorporated into soil taken from a cultivated field (FC) and a field edge (FE). These soils had contrasting native SOM qualities, with organic C contents of 0.9 and 2.5%, respectively. The amended soils were incubated under controlled environment conditions before the metabolic characteristics of the soil microbial community were determined by analysis of the activities of 19 hydrolytic enzymes, by substrate utilization within Biolog GN microplates, and C and N mineralization dynamics. For enzyme and Biolog data, metabolic diversity and community level physiological profiles (CLPP) were determined by calculating Shannons diversity index and performing canonical variate analysis, respectively. Soil type significantly affected mineralization of N from the residues, although the size and direction of the effect varied according to the crop residue material added. Both enzyme and Biolog metabolic diversity were affected by the type of crop residue incorporated. Enzyme diversity was higher in FE relative to FC soil, but soil type had no effect on metabolic diversity recorded in Biolog microplates. There were significant interactions between soil type and crop residue material for respiration, N mineralization and enzyme diversity. During the early stages of decomposition, there were similarities in the response of enzyme and Biolog CLPP to crop residue quality and soil type. In the high OM soil, there was evidence for convergence of CLPP in treatments receiving low and high quality crop residue types. However, in the low SOM soil, CLPP of low and high quality crop residue treatments were clearly different. The length of time required for the CLPP of residue amended soil to converge with that of unamended control soil depended on both residue and soil type. We conclude that both crop residue and SOM quality can affect the functional diversity of the soil microbial community, and that enzyme and Biolog analyses reflect complementary, but not inter-changeable, analyses.

Interaction of biochemical quality and particle size of crop residues and its effect on the microbial biomass and nitrogen dynamics following incorporation into soil

Abstract Mineralization of N from organic materials added to soil depends on the quality of the substrate as a carbon, energy and nutrient source for the saprophytic microflora. Quality reflects a combination of biochemical and physical attributes. We investigated how biochemical composition interacts with particle size to affect the soil microflora and N dynamics following incorporation of crop residues into soil. Four fresh shoot and root crop residues were cut into coarse and fine particle sizes, and incorporated into sandy-loam soil which was incubated under controlled environment conditions for 6 months. In the case of the highest biochemical quality material, potato shoot (C/N ratio of 10 : 1), particle size had no effect on microbial respiration or net N mineralization. For lower biochemical quality Brussels sprout shoot (C/N ratio of 15 : 1), reducing particle size caused microbial respiration to peak earlier and increased net mineralization of N during the early stages of decomposition, but reduced net N mineralization at later stages. However, for the lowest biochemical quality residues, rye grass roots (C/N ratio of 38 : 1) and straw (C/N ratio of 91 : 1) reducing particle size caused microbial respiration to peak later and increased net immobilization of N. For Brussels sprout shoot, reducing particle size decreased the C content and the C/N ratio of residue-derived light fraction organic matter (LFOM) 2 months following incorporation. However C and N content of LFOM derived from the other materials was not affected by particle size. For materials of all qualities, particle size had little effect on biomass N. We conclude that the impact of particle size on soil microbial activities, and the protection of senescent microbial tissues from microbial attack, is dependant on the biochemical quality of the substrate.

Comparison of the effects of nitrogen fertilizer on the yield, nitrogen content and quality of 21 different vegetable and agricultural crops.

The effect of level of N fertilizer on the composition, yield and quality of 21 crops was studied in experiments on adjacent sites of the same field to aid in the development of fertilizer recommendations. Yield of each of the crops first increased and then either remained the same or declined with further increases of N fertilizer. Interpretation by means of a simple model enabled response curves to be characterized by two parameters; one representing the beneficial component of the response and the other the detrimental component. Both varied greatly from crop to crop. The magnitude of the beneficial component of the response of most non-leguminous crops was largely determined by the potential demand of the crop for nitrogen; the exceptions were some root crops which responded less than would be expected on this basis. The adverse component was serious with root crops and those crops that are in the soil for only a short period. High levels of N increased the ratio of foliage to storage root dry weights even when total dry matter was unaffected. The changes were associated with a considerable increase in the % N in the dry matter of the roots. When crops were grown with their optimum levels of N fertilizer a simple linear. relationship between the mean %N in the dry matter and the total weight of dry matter per unit area covered all crops. Simple relationships also existed between total dry matter of non-leguminous crops and ( a ) the amount of N taken up by the crop from unfertilized soil, ( b ) the recovery of added fertilizer by the crop and ( c ) the beneficial component of the response of crops harvested before October. Percentage N in the dry matter at harvest was not a sensitive indicator of the extent to which plant growth was restricted by lack of nitrogen; a difference of 0·1% N in the plant material was associated with a 10% increase in yield. N fertilizer levels influenced the % dry matter and the incidence of crop disorders such as rotten roots and tissue discoloration, but the effects were seldom appreciable with practicable levels of fertilizer application.

Comparison of the effects of potassium fertilizer on the yield, potassium content and quality of 22 different vegetable and agricultural crops.

Sixty-one experiments with 15 levels of K fertilizer in the presence of excess N and P fertilizer were carried out on adjacent sites of the same field. Yield was always related to level of K fertilizer by a ‘diminishing returns’ type curve, and a derived equation, which defined relative responsiveness in terms of a single parameter, fitted the data for each crop very satisfactorily. Although the responsiveness of many of the crops was similar there were marked differences and the optimum levels of K (defined as the level at which a further 10 kg/ha increased yield by 1%) varied from 0 to 360 kg/ha, depending on the crop. Responsiveness was largely independent of the plant family to which the crop belonged, but was related to the mean plant weight atharvest; the larger the weight the less reponsive the crop. No general relation existed between responsiveness and duration of growth. The % K in the dry matter of leaves (including stems) at harvest of crops receiving the optimum levels of K fertilizer was mainly determined by the family. It was generally between 0·9 and 1·1 for the Amaryllidaceae, between 1·1 and 1·2 for the Leguminosae and between 1·9 and 2·5% for the Cruciferae. The difference between the % K in the dry matter with the optimum level of K fertilizer and that with no fertilizer was proportional to responsiveness. Percentage K at harvest was a good indicator of the extent to which crop growth was restricted by lack of potassium. At harvest crops receiving the optimum levels of K fertilizer contained between 29 and 220 kg/ha of K, but uptake increased asymptotically to a maximum as K applications were raised to higher levels. Maximum uptake for nearly all crops was almost double the uptake with the optimum fertilizer application. Percentage recovery of 100 kg/ha of added K fertilizer varied between 8 and 70%, roughly in proportion to the total crop dry weight, which varied between 1 and 15 t/ha. Effects of level of K fertilizer on crop quality were also measured and over the practical range of applications the effects were generally small. The differences between the K requirements of crops are discussed and it is argued that the responsiveness of one crop relative to that of another would be expected to be similar on a range of soils.

Comparison of the effects of phosphate fertilizer on the yield, phosphate content and quality of 22 different vegetable and agricultural crops

Fifty-six experiments, each with 15 levels of P fertilizer in the presence of excess N and K fertilizer, were carried out on adjacent sites of the same field where the soil was maintained at the same low P status. Yields, in every experiment where there was a response, were related to level of P fertilizer by a diminishing-retums type curve, and fitted an inverse polynomial equation with a single parameter to define responsiveness. Responsiveness of many crops were similar but there were, nevertheless, considerable inter-crop differences. Applications of P fertilizer increased the % P in the dry matter of lettuce and spinach as well as yields. They increased the % P in the Cruciferae and Chenopodiaceae without appreciably affecting yield. Conversely, theyhad little effect on the % P of leeks, onions, broad beans and French beans but increased yields. When the optimum levels of P fertilizer were applied, % P (in the entire plant) of the different crops was negatively correlated with total dry weight per unit area and total uptake of P was related by a single curved relationship to total dry weight. In addition, the difference between the % P in the foliage and in the storage roots of the various root crops was asymptotically related to mean plant weight. Percentage recovery of added P (100 kg/ha) by the different crops was largely determined by the total weight of dry matter. It varied from 1% when crop dry weight was 2 t/ha to 12% when it was 15 t/ha. Applications of phosphate suppressed leaf scorch of spinach. On occasion they alleviated stem rot in summer cabbage and influenced the bolting of onions and the number of defective Brussels sprouts. Otherwise, the effects on quality were small.

Chemical characterisation of vegetable and arable crop residue materials : a comparison of methods

Although it is widely recognised that chemical composition controls the patterns of decomposition and N mineralisation of crop residue materials, there has been little agreement as to the nature of the most important chemical fractions. We investigated whether this could be attributed to differences in methodologies employed for chemical characterisation of the lignin and cellulose fractions of plant materials. The cellulose and lignin contents of cauliflower, potato, red beet, Brussels sprouts and wheat crop residues were analysed by a number of contrasting methods. These were forage fibre and forest products analyses, which utilise KMnO4 and H2SO4 respectively to separate the two fractions, and a third method, which employs NaClO2. For all the materials, the forage fibre method gave substantially lower amounts of both lignin and cellulose than the other methods. There was correlation between lignin determined by the different methods. Low recovery of lignin by the forage fibre method was found to arise partly from incomplete delignification by KMnO4. The cellulose contents given by the different methods were highly correlated. However, it was apparent that the forage fibre method underestimated cellulose, since only alpha-cellulose was measured.

Screening for genotype and environment effects on nitrate accumulation in 24 species of young lettuce

BACKGROUND Nitrate accumulates in plants in response to N supply, aerial environment (predominantly light), and genotype. This paper characterises the effects of genotype, environment, and their interactions on nitrate accumulation by 24 cultivated and wild lettuce accessions grown hydroponically in winter and summer. The results will inform future strategies for selecting for low-nitrate varieties. RESULTS A preliminary study in which two accessions were sampled for nitrate over time showed largest differences between cultivars in the early-middle period of growth. Sampling the whole population of lettuce at this stage revealed significant effects of genotype, environment (with nitrate concentrations generally higher in winter), and genotype × environment interactions (largely due to a wider range of concentrations in summer). Changes in the ranking of accessions for nitrate accumulation between the two growing seasons were generally small for cultivated morphotypes. Shoot nitrate concentrations and water contents were positively associated, particularly in summer when separate relationships for different cultivated morphotypes (butterhead, cos/Romaine, crisp, leaf, and stem lettuce) were detected. Expressing nitrate concentration on either a shoot fresh or dry matter basis had relatively little effect on the ranking of most cultivated accessions, but not for the wild types. CONCLUSION There is a well-defined sampling window when differences in nitrate accumulation between lettuce genotypes are at a maximum. Delaying sampling may allow morphological changes in head form to mask earlier genotypic differences. Genotype × environment interactions are predominantly of the non-crossover type and have only a small effect on changes in the ranking of accessions between seasons, allowing selections to be made at any time of year. At least part of the genotypic variation in nitrate accumulation is associated with differences in shoot water content.

ISO-OSMOTIC REGULATION OF NITRATE ACCUMULATION IN LETTUCE

Concerns about possible health hazards arising from human consumption of lettuce and other edible vegetable crops with high concentrations of nitrate have generated demands for a greater understanding of processes involved in its uptake and accumulation in order to devise more sustainable strategies for its control. This paper evaluates a proposed iso-osmotic mechanism for the regulation of nitrate accumulation in lettuce (Lactuca sativa L.) heads. This mechanism assumes that changes in the concentrations of nitrate and all other endogenous osmotica (including anions, cations and neutral solutes) are continually adjusted in tandem to minimize differences in osmotic potential of the shoot sap during growth, with these changes occurring independently of any variations in external water potential. The hypothesis was tested using data from six new experiments, each with a single unique treatment comprising a separate combination of light intensity, nitrogen (N) source (nitrate with or without ammonium) and nitrate concentration carried out hydroponically in a glasshouse using a butterhead lettuce variety. Repeat measurements of plant weights and estimates of all of the main soluble constituents (nitrate, potassium, calcium, magnesium, organic anions, chloride, phosphate, sulphate and soluble carbohydrates) in the shoot sap were made at intervals from about 2 weeks after transplanting until commercial maturity, and the data used to calculate changes in average osmotic potential in the shoot. Results showed that nitrate concentrations in the sap increased when average light levels were reduced by between 30 and 49% and (to a lesser extent) when nitrate was supplied at a supra-optimal concentration, and declined with partial replacement of nitrate by ammonium in the external nutrient supply. The associated changes in the proportions of other endogenous osmotica, in combination with the adjustment of shoot water content, maintained the total solute concentrations in shoot sap approximately constant and minimized differences in osmotic potential between treatments at each sampling date. There was, however, a gradual increase in osmotic potential (i.e., a decline in total solute concentration) over time largely caused by increases in shoot water content associated with the physiological and morphological development of the plants. Regression analysis using normalized data (to correct for these time trends) showed that the results were consistent with a 1:1 exchange between the concentrations of nitrate and the sum of all other endogenous osmotica throughout growth, providing evidence that an iso-osmotic mechanism (incorporating both concentration and volume regulation) was involved in controlling nitrate concentrations in the shoot.

Field measurements of sap and soil nitrate to predict nitrogen top-dressing requirements of brussels sprouts

During 1983–5, 46 trials were conducted on commercial vegetable crops in Eastern England to test the value of sap and soil nitrate tests for predicting N top‐dressing requirement. This paper summarises the results of the 20 brussels sprout trials. Optimum rates of top‐dressed N (Nopt) showed no simple relationship with sap or soil nitrate measured just before top‐dressing, nor with an index based on previous cropping, or the growers’ own practice. A multiple regression of Nopt on soil nitrate (0–60 cm) and an estimate of mineralization following top‐dressing accounted for 30% of the variance of Nopt. Consideration of average yield deficits resulting from various strategies shows that, from the point of view of growers’ profits, the safest approach is to apply a top‐dressing of 120 kg N. ha‐1 in all circumstances.

Genotype and environment effects on nitrate accumulation in a diversity set of lettuce accessions at commercial maturity: the influence of nitrate uptake and assimilation, osmotic interactions and shoot weight and development

BACKGROUND The causes of the natural variation in nitrate accumulation and associated traits are studied using a diverse population of 48 mature lettuce accessions grown hydroponically in winter and summer seasons. Information on the effects of genotype, environment and their interactions will inform future selection strategies for the production of low-nitrate varieties more suited to meeting EU requirements for harvested produce. RESULTS The effects of genotype (G), environment (E) and G × E interactions were all significant, with nitrate concentrations lower but covering a wider range in summer. Concentrations of nitrate-N were positively correlated with those of water and total-N and negatively with assimilated-C in the shoot in both seasons, with all relationships partitioned according to morphotype and/or seasonal type. Corresponding relationships between nitrate-N and assimilated-N or with shoot fresh or dry weight were generally weak or inconsistent. Nitrate concentrations at an early growth stage were strongly related to those at maturity in winter, but not in summer when light levels were less variable. CONCLUSION The effects of genotype and environment on nitrate accumulation in lettuce are strongly influenced by morphotype, with most G × E interactions between accessions within the same morphotype predominantly of the non-crossover type. All low-nitrate-accumulating genotypes have increased concentrations of organic solutes (concentration regulation) and reduced water (volume regulation) to help stabilise osmotic potential within the shoots. Variability in nitrate accumulation arises more from differences in uptake than in efficiency of its chemical reduction. Genotypic differences in nitrate accumulation can be masked by changes in head morphology during maturation, provided that they are not confounded by substantial changes in intercepted light. Recent selection strategies do not appear to have produced lower-nitrate-accumulating cultivars.