Background Phosphorus (P) is a limiting nutrient in many agroecosystems and costly fertilizer inputs can cause negative environmental impacts. Cover crops constitute a promising management option for sustainable intensification of agriculture. However, their interactions with the soil microbial community, which is a key driver of P cycling, and their effects on the following crop, have not yet been systematically assessed. Scope We conducted a meta-analysis of published field studies on cover crops and P cycling, focusing on plantmicrobe interactions. Conclusions We describe several distinct, simultaneous mechanisms of P benefits for the main crop. Decomposition dynamics, governed by P concentration, are critical for the transfer of P from cover crop residues to the main crop. Cover crops may enhance the soil microbial community by providing a legacy of increased mycorrhizal abundance, microbial biomass P, and phosphatase activity. Cover crops are generally most effective in systems low in available P, and may access 'unavailable' P pools. However, their effects on P availability are difficult to detect by standard soil P tests, except for increases after the use of Lupinus sp. Agricultural management (i.e. cover crop species selection, tillage, fertilization) can improve cover crop effects. In summary, cover cropping has the potential to tighten nutrient cycling in agricultural systems under different conditions, increasing crop P nutrition and yield.
The present paper reports on three sets of experiments exploring the persistence of seeds of oilseed rape (Brassica napus). The first, where known numbers of seeds were buried in September 1991 in two field experiments, demonstrated substantial initial losses of seeds, such that only 0·2 and 3·8% of seeds were still present after 4 months. In these experiments, which were not disturbed by mechanical cultivation, there was little evidence of further decline over the following 13 months. In the second of the two experiments, seeds were then left undisturbed for a further 136 months. A mean of 1·8% of seeds were still present after this period, providing further confirmation of the lack of decline in seed numbers in these undisturbed conditions. In the second pair of experiments, known numbers of seeds of three rape cultivars were broadcast onto plots and then either ploughed into the soil immediately after the start of the experiments, or were exposed to weekly shallow tine cultivation followed by ploughing after 4 weeks. The former created a larger seedbank than the latter. The experiments were then ploughed, annually (Expt 1) or at less frequent intervals (Expt 2); appreciable numbers of seeds survived for 65 months in both. Calculations based on exponential decline curves indicated that 95% seed loss would take 15–39 months, depending on the site, cultivar and initial post-harvest stubble treatment. The third part of the paper is based on more detailed studies of persistence of seeds of six cultivars in Petri dishes and buried in 25 cm pots. This work confirmed that cultivars differed in their persistence, as Apex was confirmed as highly persistent, whereas Rebel was short-lived. There were inconsistencies in the response of cultivar Synergy between the Petri-dish and pot experiment, which need further study. This experiment also reinforced the conclusion of the initial field experiments that little seed loss occurs in the absence of cultivations. Appreciable numbers of rape seeds will persist up to 4 years, in normal cropping conditions and in the absence of cultivation one experiment has confirmed persistence for over 11 years.
SU MMARYSeeds of oilseed rape (Brassica napus L.) can persist in the soil over several years by becoming secondarily dormant and can then germinate to create volunteer plants in following crops. As well as agricultural impacts caused by volunteers, gene dispersal in time -particularly from genetically modified cultivars -can be another undesirable consequence. Conventionally bred and transgenic seeds were tested in 2001 and 2002 in laboratory experiments, and in a field experiment, by burying seeds in the soil to determine the variation in dormancy and persistence capacity.In the conventional group of cultivars tested in the laboratory, the level of dormancy was 13-76 % in 2001, and 3-76 % with an extended group in 2002. The transgenic group of cultivars was 1-31 % dormant. In the burial experiments the number of viable seeds recovered in the conventionally bred cultivars ranged from 34-90 % in 2001, and 7-68 % in 2002. In the same studies the transgenic cultivars developed persistence levels from 12-79 % in 2001, and 46-67 % in 2002. Since dormancy levels of conventionally bred cultivars from 2 harvest years in the laboratory tests correlated significantly (r=0 . 71), it appears that there is a genetic background to secondary dormancy. There was also a significant correlation (r=0 . 61 in 2001 and 0 . 80 in 2002) between the results from laboratory and burial experiments. This indicates that the laboratory approach can simulate the situation in the field. Ageing over 6 months decreased the capacity for seed persistence to about a fifth of the level shown when freshly harvested. As a consequence of ageing and environmental impacts on persistence, only seeds from the same location and harvest year should be used for testing genetic variability. The high genetic variability among currently available rape seed cultivars gives breeding strategies a good chance of ideotyping low persistence genotypes and minimizing the risk of gene dispersal in time.
Laboratory studies on the biology of oilseed rape (Brassica napus L.) showed that the induction of secondary dormancy is influenced by light environment, time of exposure to light and darkness, temperature regime and genotype. Seeds did not become dormant while exposed to light but were increasingly likely to become dormant the longer they were exposed to water stress and darkness. Dormancy was broken by alternating warm and cold temperatures.Conclusions from results obtained in Petri dishes have been tested in the field and hypotheses regarding the effects of post-harvest cultivation have been proposed. In July 1995, field experiments were initiated on a flinty silty clay loam and a sand to test the implications of post-harvest cultivation on the development of a persistent seedbank. The results largely confirmed assumptions made on the basis of laboratory findings. Seeds that had been exposed to water stress and darkness for longest, by cultivating the soil at the beginning of the experiment, immediately after seed distribution, exhibited the highest persistence rates. Seeds that were exposed to light for 4 weeks and then incorporated into the soil built up a much smaller seedbank. The seedbank was very small or nonexistent in plots that had not been cultivated at all.
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