Two different agricultural tillage practices were used to study how changes in the structure of the soil-litter system affected litter decomposition rates, microbial community composition, and soil organic matter dynamics. Surface straw placement results in spatial separation of carbon-rich litter (C:N ratio 80:1) and mineralized soil nitrogen. In contrast, when the litter is plowed into the soil, straw carbon and soil nitrogen are in intimate contact. Our field studies in Colorado showed that fungal biomass in surface- straw treatments was 144% of that in the incorporated-straw treatments, probably because fungi, with their extensive hyphal networks, are able to utilize both the surface straw carbon and the available soil nitrogen. Field studies using '4C-labeled wheat straw showed that a greater proportion of added 14C was retained in the surface-straw treatment than in the incorporated-straw treatment. Maximum net N immobilization was higher and litter de- composition was slower in the surface straw than in the incorporated straw placements both with and without experimental nitrogen addition. Slower litter decomposition of the surface litter may contribute to reduced soil organic matter losses. Soil organic matter losses may also be reduced in no-till systems as a result of the increase in the ratio of fungal to bacterial activity because of the greater growth efficiency of fungi and the accumulation of carbon in the less decomposable fungal biomass. The surface placement of straw in no-till agriculture allowed management of microclimate and microbial populations so that losses of soil organic matter and nutrients were minimized.

https://www.jstor.org/stable/pdfplus/1939274.pdf

Litter placement effects on microbial and organic matter dynamics in an agroecosystem

Changes in soil fertility caused by various organic and N-fertilizer amendments were studied in a long-term field trial mostly cropped with cereals. Five treatments were included: (I) fallow, (II) cropping with no C or N addition, (III) cropping with N-fertilization (80 kg ha −1 yr−1), (IV) cropping with straw incorporation (1800kg Cha−1 yr−1) and N-fertilization (80 kg ha−1yr−1), and (V) cropping with addition of farmyard manure (80 kg N + 1800kg Cha−1yr−1). The treatments resulted in soil organic matter contents ranging from 4.3% (I) to 5.8% (V). Microbial biomass and activity were determined by chloroform fumigation, direct counting of fungi (fluorescein diacetate (FDA)-staining and Jones-Mollison agar-film technique) and bacteria (acridine orange staining), most probable number determinations of protozoa, esterase activity (total FDA hydrolysis) and respiration. Both biomass estimates and activity measurements showed a highly significant correlation with soil organic matter. Microbial biomass C ranged from 230 to 600 μg C g−1 dry wt soil, as determined by the fumigation technique, while conversions from direct counts gave a range from 380 to 2260 μg C. Mean hyphal diameters and mean bacterial cell volumes decreased with decreasing soil organic matter content.

Microbial biomass and activity in an agricultural soil with different organic matter contents

The role of the microbial biomass in the transformation of organic matter in soil is a crucial one and the rates of turnover and mineralization of organic substrates are largely governed by the activity of the soil biomass. Inhibition of microbial activity by a low or high temperature, drought, waterlogging, extremes of pH or xenobiotic substances may result in the persistence in soil of potentially decomposable and mineralizable compounds with subsequent effects on fertility, nutrient cycling and soil structure. The microbial biomass is itself part of the soil organic matter, typically about two percent of the total organic C110 and is defined as the living microbial component of the soil and includes bacteria, actinomycetes, fungi, protozoa, algae and microfauna. Usually, plant roots and fauna larger than 5x103 μm3, such as earthworms, are not included. For the purpose of this chapter, the biomass will be considered as an undifferentiated whole, usually expressed quantitatively as biomass-C., cell biomass or biovolume (μm3). However, the activities of various components of the biomass — protozoa, nematodes, etc. — have been included where this seems appropriate.

The Soil Biomass

Decomposition of leaves of smooth cordgrass (Spartina alterniflora Loisel.) was monitored for two cohorts of leaves from September 1984 to May 1985 (autumn and winterspring) at Sapelo Island (31°23′ N; 81°17′ W). The leaves were tagged in plance at the ligule, rather than cut and placed in litterbags. Dead leaves were not abscised from shoots. Loss of organic mass from the attached leaves was at least 60 to 68% of the orginal values. Fungal mass, as measured by an enzyme-linked immunosorbent assay, formed > 98% of the microbial standing crops in two of three autumn samples, and in all samples for the colder, drier, winterspring cohort. Fungal mass was probably mostly in the form of the mycelium and pseudothecia of an ascomycete, Phaeosphaeria typharum (Desm.) Holm. Fungal dominance of microbial standing crops declined when autumn leaves bent downward and acquired a large sediment content (ash=35% of dry matter); the bacterial crop then rose to 7% of the total microbial crop. Microphotoautotrophic mass was always measurable, but was never more than 2% of the microbial crop. Carbon-dioxide fixation was much lower than carbon-dioxide release, and a substantial portion of the fixation may have been anaplerotic fungal fixation. Threeto 8 wk net fungal productivity (average per day) was much greater (16 to 26 times) than measured instantaneous bacterial productivity (extrapolated to per-day values) early in each decay period. Fungal productivity was negative late in the decay period. Fungal productivity was negative late in the decay period for autumn leaves, and was approximately equal to bacterial productivity late for winter-spring leaves. Net nitrogen immobilization was observed only late in the decay period for autumn leaves, implying that nearly all dead-leaf nitrogen was scavenged into fungal mass after the first sampling interval. Flux estimates for dead-leaf carbon indicated a flow of 11–15% of the original to fungal mass, 2% to bacterial mass, 15–21% to carbon dioxide, 10–12% to dissolved leachage, and 34–36% to small particles; 32–39% remained attached as shreds at the end of the study periods. Salt-marsh periwinkles (Littorina irrorata Say) appeared to be the major shredders of dead leaves and conveyors of leaf-particulate material to the marsh sediment, at least in those parts of the marsh where the snails are densely concentrated (usually areas of short- and intermediateheight cordgrass shoots).

Decomposition and microbial dynamics for standing, naturally positioned leaves of the salt-marsh grass Spartina alterniflora

Specifically radiolabeled [14C-lignin]lignocellulose and [14C-polysaccharide]lignocellulose from the salt-marsh cordgrass Spartina alterniflora were incubated with an intact salt-marsh sediment microbial assemblage, with a mixed (size-fractionated) bacterial assemblage, and with each of three marine fungi, Buergenerula spartinae, Phaeosphaeria typharum, and Leptosphaeria obiones, isolated from decaying S. alterniflora. The bacterial assemblage alone mineralized the lignin and polysaccharide components of S. alterniflora lignocellulose at approximately the same rate as did intact salt-marsh sediment inocula. The polysaccharide component was mineralized twice as fast as the lignin component; after 23 days of incubation, ca. 10% of the lignin component and 20% of the polysaccharide component of S. alterniflora lignocellulose were mineralized. Relative to the total sediment and bacterial inocula, the three species of fungi mediated only very slow mineralization of the lignin and polysaccharide components of S. alterniflora lignocellulose. Experiments with uniformly 14C-labeled S. alterniflora material indicated that the three fungi and the bacterial assemblage were capable of degrading the non-lignocellulosic fraction of S. alterniflora material, but only the bacterial assemblage significantly degraded the lignocellulosic fraction. Our results suggest that bacteria are the predominant degraders of lignocellulosic detritus in salt-marsh sediments.

Relative contributions of bacteria and fungi to rates of degradation of lignocellulosic detritus in salt-marsh sediments.

Direct-count analysis of fungal biomass cannot be accurate or precise without firm knowledge of values for quantity of mass per unit volume of mycelium. We have determined a range in weight of dry matter, carbon and nitrogen per cm3 of fresh mycelium for salt-marsh fungi, using microecosystems which were rough simulations of a range of natural environments of fungi in salt marshes dominated by Spartina alterniflora Loisel. We found a wide range in dry matter per cm3 of mycelium, averaging 0.2 g cm3 for a group of less dense samples, and 0.9 g cm3 for more dense samples. The high densities were associated with young mycelium of two species known to be well adapted to the salt-marsh environment. The low densities were found for old mycelium, and for young and old mycelium of a species whose degree of adaptation to the salt-marsh environment is uncertain. Nitrogen and carbon percentages of dry fungal biomass were similar to those reported by other investigators (nitrogen, -4%; carbon, -35%).

Factors for Conversion of Fungal Biovolume Values to Biomass, Carbon and Nitrogen: Variation with Mycelial Ages, Growth Conditions, and Strains of Fungi from a Salt Marsh

S. Y. Newell

Papers

Factors for Conversion of Fungal Biovolume Values to Biomass, Carbon and Nitrogen: Variation with Mycelial Ages, Growth Conditions, and Strains of Fungi from a Salt Marsh