Successful use of compost to maintain plant health and soil fertility requires consistent monitoring of compost quality. For this purpose, near infrared (NIR) spectroscopy might be a useful alternative to standard procedures which are often time-consuming and laborious. Ninety-eight yard-waste compost samples were analysed by conventional methods and NIR spectroscopy. Reference analysis included the determination of age, organic C (C org ) and total N (N t ) contents, C / N ratio, microbial biomass (C mic ), the ratio of C mic to organic C (C mic / C org ), basal respiration, metabolic quotient (qCO 2 ), hydrolysis of fl uorescein diacetate (FDA-HR), specifi c enzyme activity, i.e. FDA-HR related to C mic , and suppression of pathogens. All samples were scanned in the visible light and near infrared regions (400-2500 nm). Cross-validation equations were developed using the whole spectrum (fi rst and second derivative) and a modifi ed partial least-square regression method. NIR predicted basal respiration and age successfully [ratio of standard deviation and standard error of cross-validation (RPD) was 4.3 or 2.9, respectively]. All other properties, i.e. C org and N t contents, C / N ratio, C mic , C mic / C org , qCO 2 , FDA-HR, specifi c enzyme activity and suppression of pathogens at an inoculation level of 5‰ related to rating or fresh weight, respectively, were predicted with moderate success (1.4 ≤ RPD ≤ 2.0). However, the coeffi cients of determination for specifi c enzyme activity and suppression of pathogens related to fresh weight were rather low (r² = 0.49 and 0.47, respectively). The results presented indicate that NIR spectroscopy is able to determine important compost quality parameters. However, further research is needed concerning the basis of and limitations for the determination of specifi c enzyme activity and suppressiveness by NIR spectroscopy.
A pot experiment was carried out (1) to compare C and N yield of different plant parts, nutrient concentrations, and root colonization between the non-mycorrhizal mutant P2 (myc − ) and the symbiotic isoline Frisson (myc + ), (2) to investigate the effects of arbuscular mycorrhizal fungi and growing pea plants on microbial decomposition of 15 Nlabeled maize residues, and (3) to follow the distribution of the added substrate over different soil fractions, such as particulate organic matter, soil microbial biomass, and microbial residues. Yields of C in straw, grain, and roots of myc + peas were significantly higher by 27%, 11%, and 92%, respectively, compared with those of myc − peas. The δ 13 C values in the different plant parts were significantly higher in myc + than in myc − tissue with and without maize. Application of labeled maize residues generally resulted in 15 N enrichment of pea plants. At the end of the experiment, the ergosterol concentration in roots of mature peas did not differ between the two isolines, indicating similar colonization by saprotrophic fungi. The decomposition of added maize residues was significantly reduced by the myc − peas, but especially by myc + peas. The formation of microbial residue C was increased and that of microbial residue N was reduced in the presence of plants. The insufficient N supply to soil microorganisms reduced decomposition of maize residues in the presence of peas, especially myc + peas.
Providing an appropriate negative control for the experimental factor arbuscular mycorrhiza (AM) is a fundamental methodological problem. Therefore, the nonmycorrhizal (myc –) and nonnodulating (nod –) pea (Pisum sativum L.) mutant P2 was studied together with the parental symbiotic isogenetic variety Frisson in three experiments: (1) growth response to water supply in a climate chamber under nonsymbiotic growth conditions, (2) field evaluation at three sites in the Alentejo, South Portugal, and (3) growth response to P supply in a soil low in available P in a greenhouse‐chamber experiment. In the climate chamber at high NPK levels, mutant P2 achieved the same biomass as Frisson at 80% and 40% water‐holding capacity, respectively. For the field evaluation, three sites were chosen with normal arable use (Évora), extensive use as Montado (Portel), and intensive horticultural use (Mitra). The colonization of pea roots with AM fungi ranged from 4% (Mitra) to more than 90% (Portel), probably caused by differences in P availability. The plant density of mutant P2 was generally 25% lower than that of Frisson. Yield indices were all lowest at Portel, despite the same NPK fertilization. Grain and shoot yield of mutant P2 did not reach the level of Frisson at any site. Differences in N and P concentrations between the two isolines were insignificant in most cases. Differences in the amount of shoot P per plant consistently mirrored the mycorrhizal status of the three sites. Roughly 50% of the yield depression per m2 could be attributed to the lower plant density of mutant P2, the remaining 50% must be caused by AM‐fungal colonization or other factors. In the final pot experiment using the soil with low P availability from Portel, the main benefit of AM for peas was enhanced P uptake. Central questions could not be answered using a nonmycorrhizal control. However, mutants remain one interesting tool, best be used in combination with other approaches to estimate the effects of mycorrhization.
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