E. Bremer

Microbial utilization of 14C[U]glucose in soil is affected by the amount and timing of glucose additions

Microbial utilization of glucose-14C by soil microbes was investigated in two laboratory experiments. In the first experiment, 14C-labelled glucose was added to two soils at seven rates ranging from 36 to 2304 μg C g−1 soil. An average of 42% of added 14C was mineralized by day 3 at glucose rates ⩾288 μgCg−1 soil in both soils, but this proportion declined at lower rates. Only 30% of added 14was mineralized at the lowest rate of glucose addition. The fraction of soil 14C released by fumigation-extraction (FE-14C) ranged from 11 to 36%, and decreased linearly with the proportion of 14C mineralized in both soils. The effects of addition rate persisted until the end of the experiment, at 35 days. In the second experiment, addition of unlabelled glucose at 6, 24 or 72 h after addition of glucose-14C at 30 μg C g−1 soil did not appreciably affect the proportion of 14C mineralized, while addition 72 h before or immediately after 14C addition increased 14C mineralization by about 50%. Fumigation-labile 14C was reduced in all cases by addition of unlabelled glucose, with the greatest reduction when unlabelled glucose was added immediately after glucose-14C. We conclude that assimilated glucose-14C was incompletely metabolized at low rates of glucose addition unless soil microorganisms were ‘activated’ by a prior addition of glucose. The proportion of 14C mineralized at low rates of glucose addition to soils may be useful as an indicator of C availability: a small proportion mineralized may indicate low C availability whereas a large proportion mineralized may indicate high C availability.

Extractability of microbial 14C and 15N following addition of variable rates of labelled glucose and (NH4)2SO4 to soil

The objective of this study was to investigate the effect of variable rates of substrate addition and length of incubation period on in situ estimates of the proportion of microbial C and N released by a chloroform-fumigation-extraction method. A Bradwell silty loam soil containing ca 340 μg microbial C g−1 soil was amended with [14C(U)]glucose and [15N](NH4):SO4 at 30 and 2. 300 and 2, and 300 and 20μg of C and N. respectively, g−1 soil. The following measurements were taken at 0.25, 0.5. 1. 3 and 7 days after the addition of substrates: respired 14C, mineral 15N, total 15N and increase in extractable 14C and 15N upon fumigation. 14C-efficiency, defined as the proportion of 14C in the microbial biomass relative to the total amount of 14C removed from the soil solution, was >70% in all treatments at 0.25 and 0.5 days and in the treatment receiving the lowest rate of glucose at all sampling times. The proportion of microbial 14 released by fumigation-extraction was >40% when the efficiency was >70%. but was 34% when the efficiency was lower. In the treatment receiving the low rate of glucose microbial N accounted for ca 50% of added 15N, of which >40% was released by fumigation-extraction; in the treatments receiving the high rate of glucose microbial N accounted for an average of 81% of added 15N, of with only 24% was released by fumigation-extraction at 3 and 7 days after the addition of substrates. Based on these observations, it was concluded that at low rates of glucose addition microbial 14C and 15N remained present among cytoplasmic constituents and little if any was utilized in biosynthesis; sufficient glucose to stimulate microbiul growth and a sufficient period of incubation would be required to obtain reliable estimates of the proportion of microbial C and N released upon fumigation-extraction.

Carbon dioxide evolution from wheat and lentil residues as affected by grinding, added nitrogen, and the absence of soil

SummaryA study was conducted to determine the effects of grinding, added N, and the absence of soil on C mineralization from agricultural plant residues with a high C:N ratio. The evolution of CO2 from ground and unground wheat straw, lentil straw, and lentil green manure, with C:N ratios of 80, 36, and 9, respectively, was determined over a period of 98 days. Treatments with added N were included with the wheat and lentil straw. Although the CO2 evolution was initially much faster from the lentil green manure than from the lentil or wheat straw, by 98 days similar amounts of CO2 had evolved from all residues incubated in soil with no added N. Incubation of plant residues in the absence of soil had little effect on CO2 evolution from the lentil green manure or lentil straw but strongly reduced CO2 evolution from the wheat straw. Grinding did not affect CO2 evolution from the lentil green manure but increased CO2 evolution from the lentil straw with no added N and from the wheat straw. The addition of N increased the rate of CO2 evolution from ground wheat straw between days 4 and 14 but not from unground wheat straw, and only slightly increased the rate of CO2 evolution from lentil straw during the initial decomposition. Over 98 days, the added N reduced the amounts of CO2 evolved from both lentil and wheat straw, due to reduced rates of CO2 evolution after ca. 17 days. The lack of an N response during the early stages of decomposition may be attributed to the low C:N ratio of the soluble straw component and to microbial adaptations to an N deficiency, while the inhibitory effect of N on CO2 evolution during the later stages of decomposition may be attributed to effects of high mineral N concentrations on lignocellulolytic microorganisms and enzymes.

Selection of Rhizobium leguminosarum strains for lentil (Lens culinaris) under growth room and field conditions

Most of the production of lentil (Lens culinaris) on the Great Plains occurs on soils that are free of indigenous Rhizobium leguminosarum. Inoculation is required to increase yields through N2 fixation. A screening program to evaluate the effectiveness of R. leguminosarum strains for lentil was initially carried out under controlled environments followed by an evaluation under field conditions. In two separate growth room experiments, the effectiveness of 185 and 24 different strains of R. leguminosarum were tested for Laird and Eston lentil. Significant differences between strains in number of nodules, shoot weight and nitrogenase activity (acetylene reduction activity, ARA) were found for lentil grown for 5 weeks. When lentil were grown for 7 weeks, significant differences between strains in number of nodules, total plant weight, total N, and % N were observed.Fourteen strains plus Nitragin ‘C’ inoculant were selected for further field testing on Eston and Laird lentil at two locations in 1986 and one site in 1987. Inoculation increased yield up to 135%. Percent Ndfa and total N2 fixed ranged from 0 to 76 and 0 to 105 kg ha-1, respectively. N2-fixing activity was site specific and higher spring soil NO3-levels resulted in lower N2-fixing activity. Depending on site and growing conditions, strains 99A1 and I-ICAR-SYR-Le20 appeared to be superior to the other strains tested. A good agreement was found between the estimates for N2 fixation based upon the 15N-isotope dilution and the classical N difference methods. Number of nodules, dry weight of nodules and ARA of Eston and Laird lentil grown under growth room conditions failed to show positive correlations with total dry matter production, total N or total N2 fixed of field grown lentil. However, total plant weight and total N of lentil grown under growth room conditions were highly correlated with field parameters, and were the most reliable screening parameters for the selection of superior rhizobial strains.

Influence of competition for nitrogen in soil on net mineralization of nitrogen

We tested the hypothesis that plants only stimulate net mineralization of N when intense competition for N exists between plants and heterotrophs. Nitrogen mineralization in the soil used was insensitive to the range of moisture fluctuations that were inevitable during plant growth. Pots were planted to wheat (Triticum aestivum L.) or left unplanted and received no straw, straw added in one central layer, or straw added uniformly through the whole soil volume. Through the addition of15 N-labelled nitrate, initial soil inorganic N was increased to 17 μg g−1 in unplanted treatments and to 17 μg g−1 and 72 μg g−1 in planted treatments. Straw addition increased microbial immobilization of labelled N (soil inorganic N at planting), but did not reduce net mineralization of unlabelled soil N (soil organic N at planting), indicating that straw decomposers immobilized N early in the growth period. Plant growth did not reduce immobilization of N by straw decomposers. Net mineralization of N was not affected by plant growth at the low rate of N addition, but was reduced at the high rate of N addition. We conclude that the influence of wheat growth on net mineralization of N depends on soil N availability, with reductions in net mineralization at high N levels due to increased immobilization.

Assessment of reference crops for the quantification of N2 fixation using natural and enriched levels of 15N abundance

Abstract In studies with the 15 N-isotope dilution method for quantifying N 2 fixation, the importance of selecting a reference crop with a similar pattern of soil N uptake as the N 2 -fixing crop has been emphasized. Because temporal variation in the 15 N enrichment of soil inorganic N will be different following addition of 15 N-enriched and natural abundance sources of N, we hypothesized that only a valid reference crop would provide similar estimates of N 2 fixation in both cases. Barley (Hordeum vulgare L.) , flax (Linum usitatissimum L.) , and a non-N 2 -fixing pea (Pisum sativum L.) cv. were grown as reference crops for N 2 -fixing pea under greenhouse conditions. The soil was amended with either 15 N-enriched plant material or natural abundance (NH 4 ) 2 SO 4 . The amount and 15 N enrichment of soil inorganic and total plant N were determined periodically between 6 and 84 days after planting. Barley and non-N 2 -fixing pea assimilated soil inorganic N earlier than N 2 -fixing pea, while flax assimilated soil inorganic N later. The δ 15 N of soil inorganic N increased between days 14 and 36 due to isotopic fractionation during plant N uptake. By day 84 the δ 15 N of plant N was the same as initial soil inorganic N for barley and non-N 2 -fixing pea, while flax had a lower δ 15 N due to incomplete depletion of soil inorganic N. In the 15 N-enriched treatment, barley had a lower 15 N enrichment at the final sampling date than the other reference crops, even though it had obtained a higher proportion of its N earlier. This was attributed to a greater root-induced turnover of soil N in the barley treatment. Estimates of N 2 fixation were only similar in natural abundance and 15 N-enriched soils when barley was used as the reference crop. It was concluded that comparison of estimates of N 2 fixation in natural abundance and 15 N-enriched soils may provide a useful criterion for selecting a valid reference crop.