Influence of Soil Organic Matter Concentrations on Carbon and Nitrogen Activity1

L. E. WOODS AND G. E. SCHUMAN2

ABSTRACTThe relationship of soil organic matter (OM) concentration to

microbial biomass concentration and mineralizable OM is centralto understanding the establishment and functioning of soil nutrientcycles. These parameters are presumably related to abovegroundplant biomass and plant N concentration, but the mechanisms andcontrols of these interactions are not well understood. To furtherevaluate these relationships, a field study was established in a seriesof soil materials whose organic C concentrations ranged from 1 to21 g kg"1. Surface soil (0-15 cm) and vegetation samples were col-lected from plots of each treatment during the 1983 growing season.Microbial biomass was measured by chloroform fumigation and in-cubation; mineralizable C and N were measured in 20-d laboratoryincubations; plant growth was measured by weighing material clippedfrom 0.18-m2 frames; and plant N concentration was measured byKjeldahl digestion and colorimetric analysis. Microbial biomass in-creased linearly with soil OM concentration. Mineralizable N andplant production also increased with soil OM, but were greater with7 than with 15 g kg~' of organic C. Even though aboveground plantbiomass was greater with either 7 or 15 than with 21 g kg'1 organicC, plant N concentrations were highest with 21 g kg '. Soil OMconcentrations were more closely related to microbial biomass thanto mineralizable C and N, or to plant biomass and plant N concen-trations. Nitrogen mineralization in the laboratory corresponded toplant N concentration in the field. Soil OM concentrations con-trolled microbial biomass C and N concentrations. However, addi-tional factors also influenced the activity of the microbes and theresultant OM mineralization and plant N concentrations.

Additional Index Words: nutrient cycling, mineralizable N, mi-crobial biomass, chloroform fumigation, N cycling.

Woods, L.E., G.E. Schuman. 1986. Influence of soil organic matterconcentrations on carbon and nitrogen activity. Soil Sci. Soc. Am.J. 50:1241-1245.

STUDIES OF ECOSYSTEMS with large accumulationsof organic matter (OM) and nutrients in the sur-

face soil demonstrate the importance of microbially-mediated processes, but studies of less-mature eco-systems with smaller soil OM accumulations are rel-atively rare (Roberts et al, 1980; Marrs et al., 1980).Plant growth, OM decomposition, and nutrient im-mobilization and mineralization require that suffi-cient quantities of active nutrients accumulate andpersist in the soil (Anderson, 1977; Roberts et al.,1980). Without adequate pools of active nutrients,long-term fertility problems can exist, even if plantgrowth is initially satisfactory (Schuman and Power,1981). A better understanding is needed of the sizeand turnover rate of the soil microbial biomass in re-lation to the total amount of soil OM (Paul and Vo-roney, 1984).

Revegetated mined lands in south-central Wyomingprovided soils with a wide range of OM concentra-tions and the opportunity to study the influence of

1 Contribution from the USDA-ARS, High Plains Grasslands Re-search Station, Cheyenne, in cooperation with Pathfinder MinesCorp., Shirley Basin, WY. Received 24 May 1985.

2 Soil Scientists, USDA-ARS, High Plains Grasslands ResearchStation, 8408 Hildreth Road, Cheyenne, WY 82009.

this range of concentrations on nutrient cycling. Plantcommunities were established earlier in low OM ov-erburden (White River geologic material), but theyfailed to persist despite initial seedling establishmentand fertilization (Rauzi and Tresler, 1978). Plant com-munities established on adjacent topsoiled sites per-sisted, but had lower plant N concentrations than thesame species growing on nearby native rangeland(Stanley et al., 1982). Because the White River ma-terial lacked physical or chemical limitations to plantgrowth (Schuman and Taylor, 1978), we postulatedthat the low concentration of soil OM has precludedthe establishment of essential nutrient cycling pro-cesses.

Microbial biomass responds more rapidly than totalsoil organic C or N to changes that affect the inputsor decomposition of organic material (Nannipieri,1984). Differences in microbial biomass and activitymore readily influence the aboveground plant biomassand its N concentration than do soil OM concentra-tions themselves. Our objectives were to evaluate therelationship of soil OM concentrations to both the sizeand potential activity (respiration and N mineraliza-tion) of the microbial biomass, and to the above-ground plant biomass and its N content.

MATERIALS AND METHODSSite Description

Field plots were located in a semiarid shortgrass steppe ata uranium mine in the Shirley Basin of eastern Wyoming.Elevation at the site is 2200 m, long-term average annualprecipitation is 260 mm, average frost-free period is 88 d,and mean annual temperature is 3.4°C.

January and February of 1983 were warm and dry monthsfor this area. April was cold (average temperature for themonth was — 0.5°C). Temperatures were near normal forthe remainder of the growing season. Precipitation duringthe calendar year was 279 mm, which is 6% above normal;but precipitation during the growing season (June-August)was 134 mm, which is 70% above normal for that period.In spite of the above-normal precipitation during the grow-ing season, soil-water tension (0-15 cm) on all sampling dateswas below —1.5 MPa, except on 16 June.

Treatments included White River overburden withouttopsoil (WR), overburden covered with either 15 (Tl) or 30(T2) cm of topsoil, and native rangeland (NAT). White Rivermaterial is composed primarily of arkosic sands interbeddedwith fine silts and montmorillonitic clays deposited duringthe Oligocene Epoch. The native soil is a Borollic Haplargidof the fine-loamy mixed family (Young and Singleton, 1977).The surface material of the two "topsoiled" sites is a mixtureof the A and B hori/ons of the native soil. It is placed overa 1- to 2-m layer of White River material. The NAT plotswere selected in an area with limited shrub populations sothat the plant community more closely resembled those ofthe reseeded areas. Grazing by livestock was eliminated fromall plots.

Topsoil treatment Tl had 15 cm of soil placed over theWhite River material. Some mixing of the topsoil and WhiteRiver material occurred during seedbed preparation. Thisarea was seeded in 1977 with a mixture of native grasses.These plots also received 156 kg N ha~' in 1981.

Topsoil treatment T2 had 30 cm of soil placed over the

1241

1242 SOIL SCI. SOC. AM. J., VOL. 50, 1986

Table 1. Selected soil characteristics of the surface 15 cm inplots with increasing amounts of soil organic matter.

Treatment

White RiverTopsoil 1Topsoil 2Native

Organic TotalC N

—— g kg- ——1 0.077 0.42

15 1.1421 1.53

Texture

Sandy loamSandy clay loamSandy clay loamLoam

PH

7.57.47.47.5

Electricalconductivity

dS m-'0.360.880.630.98

White River material. There was less mixing with the WhiteRiver material during seedbed preparation because of thegreater soil depth in this treatment. The T2 plots were seededin 1972 with a single native grass species (Rauzi and Tresler,1978).

Major plant species in WR plots were basin wildrye [Ely-mus cinereus Scribn. & Merr.J, western [Agropyron-smithiiRybd.] and crested wheatgrasses [Agropyron cristatwn (L.)Pers]. Major plant species in Tl plots were western wheat-grass, thickspike wheatgrass [Agropyron dasystachyum(Hook.) Scribn.], needleandthread [Stipa comata Trin. &Rupr.], and Poa spp. Major plant species in T2 plots werewestern and crested wheatgrass. Major plant species in theNAT area were western wheatgrass, green needlegrass [Stipaviridula Trin.], blue grama [Bouteloua gracilis (H.B.K.) Lag.ex Griffiths], big sagebrush [Artemesia tridentata ssp.Wyomingensis Beetle & Young), and Junegrass [Koeleriacristata (L.) Pers.].

Selected soil characteristics are presented in Table 1. Thesetreatments produced soil organic C concentrations thatranged from 1 to 21 g kg~'.

Sampling ProceduresThree field plots were established in each treatment and

sampled five times from 16 June through 5 October. Thethree sets of samples were treated as replicates in the labo-ratory. Soil collected on the five sample dates was bulkedfor the baseline soil characteristics reported in Table 1. Mi-crobial biomass, respiration, and N-mineralization data werenot obtained on 16 June or 3 August. Plants were not sam-pled on 16 June.

Vegetation samples were harvested at ground level insideone randomly located quadrat (0.18 m2) within each of threereplicate plots of each treatment. Litter and surface mulchwere not collected. Plant material was oven-dried at 60°Cand weighed to estimate the aboveground plant biomass.Eight to 10 surface soil cores (15-cm deep by 2.5-cm diam)were collected from within each quadrat and combined intoa single sample. The soil samples were kept chilled, but notfro/en, for transport to the laboratory and placed in a 5°Crefrigerator overnight.

Laboratory AnalysesGeneral soil properties were determined on air-dried and

sieved (2 mm) samples composited from all sample dates.All soil results are expressed on an oven-dry basis (105°Cfor 24 h). Moisture retention at -0.033- and - 1.5-MPa soil-water potential was determined by pressure-plate extraction,total C, by a modified Walkley-Black titration (Nelson andSommers, 1982), and total N by a modified micro-Kjeldahldigestion procedure (Schuman et al., 1973) with colorimetricanalysis of NFLf (Technicon, 1973a). Nitrate and NHJ weredetermined colorimetrically (Technicon, 1973a, b) in a 1 MKC1 extract (Keeney and Nelson, 1982).

Microbial biomass C and N were determined by the chlo-roform-fumigation-incubation technique (Jenkinson andPowlson, 1976). Twenty grams of field-moist soil were placedin 50-mL Erlenmeyer flasks and moistened to approximately—0.033-MPa moisture with deionized water within 24 h of

sampling. The samples were divided into three groups, andplaced in separate vacuum desiccators containing moist pa-per towels. One group was exposed to a saturated chloro-form atmosphere (double-distilled to remove ethanol) for 24h, degassed for 2 h, placed in 0.5-L respiratory chamberswith 2 mL of 1 M NaOH, and incubated at 25°C. The chlo-roformed samples and one set of the nonchloroformed sam-ples were analyzed after 10 d and the third set was incubatedfor 20 d. Excess base was titrated to the phenolphthaleinend-point with standardized 1.0 M HC1 and the soil wasanalyzed for moisture content and 1 M KCl-extractableNO3~ and NHJ (Jenkinson and Powlson, 1976).

Microbial biomass C and N were calculated using the for-mulas of Voroney (1983). No respired C or mineralized Nfrom unfumigated soil was subtracted from that in fumi-gated soil. Microbial biomass C was calculated as C-flush/ka where kc = 0.41. Microbial biomass N was calculated asN-flush/A^, where kN = 1.86[(C-flush/N-flush)-°879] (Vo-roney, 1983). The C and N flushes are the amounts of CO2-C and mineral-N produced during 10 d of incubation fol-lowing chloroform fumigation. The use of these formulas isby no means universal but they account for net immobili-zation of N during incubation when C/N are wide.

Statistical AnalysesAnalyses of variance were performed for the available dates

and soil organic matter levels. Honestly significant differ-ences (HSD) were calculated where appropriate. Regressionsof microbial biomass C and N concentrations, respiration,and N mineralization during 20 d incubation against soilorganic C concentration were performed. Average concen-trations of the three available sample dates for the depen-dent variables were used because organic C concentrationswere obtained from combined soil samples from all dates.

RESULTS AND DISCUSSIONPrevious research at this site concluded that the low

concentrations of soil OM may be limiting the estab-lishment of functional and sustainable nutrient cycles(Schuman and Power, 1981). In 1972, grasses wereseeded on the WR and T2 soil materials. Seedling es-tablishment was adequate; however, the plants estab-lished on the WR soil material did not persist (Rauziand Tresler, 1978). Vegetation established on the T2soil materials persisted and aboveground biomass ex-ceeded that of the nearby native rangeland. However,plant N concentrations in wheatgrasses from the T2treatment were lower than in those from the nativerangeland (Stanley et al., 1982).

If lack of OM prevented plant growth from persist-ing in WR material and reduced plant N concentra-tions in T2 material, then it should also limit eitherthe microbial biomass or its activity. Further, if OMconcentration is a major controlling factor in nutrientcycling, then levels of microbial biomass and activityshould increase consistently with soil OM concentra-tions. Differences in management between the sets oftopsoiled plots could alter this relationship. The soilplaced over the spoil on the T2 plots had been stock-piled for at least 5 yr, while that on the Tl plots wasdirectly removed from an undisturbed area of themine. Although it seems unlikely that stockpiling ef-fects would persist for 10 yr, this difference mightchange the relative responses of these two treatments.In addition, the Tl plots were fertilized with 156 kgN ha"1 in 1981, but the T2 plots were not. This fer-

WOODS & SCHUMAN: SOIL ORGANIC MATTER CONCENTRATIONS ON C AND N ACTIVITY 1243

1.5-

1.0-

' 0.5-

y= 0.0623X + 0.081

r2 = O.98**

0 10 20

Soil O rga nic C (g K g " ' )

Fig. 1. Relationship between soil organic C concentrations and mi-crobial biomass C.

40-

30-

" 20-

10-

y = I . 5 4 X +0.9r z = 0.69**

10 20

Soi I Organ ic C (g kg'1)

Fig. 3. Relationship between soil organic C concentrations and min-eralizable N (20-d incubations).

tilization 2 yr before sampling could also influence theresponses of the Tl plots.

Concentrations of mineral N (the sum of NOf andNHJ) were never >36 mg kg"1, which was not sig-nificantly different from zero. In open systems likethese, low concentrations of mineral N do not indicatelack of nutrient cycling.

Micrpbial biomass C and N concentrations werelowest in WR plots, and increased linearly (r2 = 0.98)with increasing soil OM concentrations (Fig. 1 and 2).The data indicate that neither fertilizing the Tl plots,nor stockpiling the topsoil for the T2 plots, changedmicrobial biomass concentrations relative to soil OMconcentrations. The intercepts of these lines (Fig. 1and 2) suggest that there would be some microbialbiomass in the absence of soil OM; however, this isimpossible and instead indicates the need to furtherstudy the establishment of microbial populations atextremely low soil OM concentrations.

Mineralizable N (in 20 d of incubation) also in-creased with increasing soil organic C concentrations,although not as consistently as microbial biomass (r2

= 0.69) (Fig. 3). Topsoiled treatment Tl, even thoughit was lower in OM concentration, actually had greater

o 10Soil O rgan i c C ( g kg"' )

Fig. 2. Relationship between soil organic C concentrations and mi-crobial biomass N.

800-

600-

'- 400-

200-

Soi I O r g a n i c C (g kg )

Fig. 4. Relationship between soil organic C concentrations and res-piration (20-d incubations).

mineralizable N concentrations than did T2. This re-lationship is much less linear than that between mi-crobial biomass and soil OM concentrations. Perhapsfertilizing and stockpiling changed the mineralizabil-ity of the soil organic N, even though they did notaffect the microbial biomass.

Respiration during 20 d of incubation also in-creased with increasing soil OM (Fig. 4), but the twointermediate treatments did not differ from each other.Both respirable C and mineralizable N appeared todepend on other factors in addition to OM concen-trations to a greater degree than did microbial bio-mass.

Plant yields were too variable to demonstrate cleartrends except that the WR plots were essentially bar-ren (Table 2). Both Tl and T2 treatments had moreaboveground plant biomass than did native range-land. Aboveground plant biomass was greatest on 7July in all treatments, as expected in this physio-graphic region given the wet spring and seasonal tem-peratures. Thereafter, plant biomass declined steadily,except for a late season increase on the Tl plots. Thisincrease resulted from western wheatgrass respondingto the late season precipitation (14.5 mm in Septem-ber).

1244 SOIL SCI. SOC. AM. J., VOL. 50, 1986

Table 2. Effect of treatments and sampling dates on plantyields and N concentrations.

Table 3. Concentrations of N in wheatgrass (Agropyron spp.)from native and topsoil plots (live shoots only).

Treatment 6 July 3 August 7 September 5 October

Aboveground biomass, kg ha"1

White RiverTopsoil 1Topsoil 2NativeHSD(0o5) =

White RiverTopsoil 1Topsoil 2NativeHSD(0.05) =

White RiverTopsoil 1Topsoil 2NativeHSD(0.05) =

216 123 211700 1306 9311422 1276 8301220 807 688

1076Plant N concentration, g kg"1

6.012.010.214.5

4.8

1.3020.4014.5117.70

11.12

6.710.78.5

12.1

Plant0.83

13.9810.849.76

23.8T8.75.9

10.9

N, kg ha"1

0.508.104.907.60

111322

574753

5.510.44.49.5

0.0613.752.527.16

t This value is the mean of very small samples, composed exclusively of:single species of forb.

Average plant N concentrations were always greaterin vegetation from NAT than from any of the othertreatments, except for the Tl plots in October and theWR plots in September (Table 2). The October in-crease on Tl probably also reflected the new growthin western wheatgrass. The high plant N levels in theWR samples in September were for two small samplescomposed exclusively of a single forb [Sphaeralceacoccinia (Pursh) Rydb.]. Average plant N concentra-tions were lower in the vegetation from both the Tland T2 plots than in that from the NAT plots, andwere consistently less in vegetation from T2 than fromTl plots (Table 2).

Higher plant yields in the Tl treatment more thanoffset the lower plant N concentrations, and there wasconsistently more aboveground plant N per ha in Tlthan in any other treatment (Table 2). The NAT plotswere generally next highest, followed by the T2 plots.These comparisons are limited because of the differ-ences in plant species growing on the various sites. Inorder to determine if these trends simply reflected thedifferences in plant species, live shoots of wheatgrasses(Agropyron spp.) were collected separately from bothnative and topsoiled areas on 3 August and 7 Septem-ber. Plant N concentrations in these species-matchedsamples were also greater in vegetation from the na-tive sites (Table 3).

Both plant N concentrations and mineralizable Nwere highest in NAT plots, next highest in Tl plots,followed by the T2 plots, and lowest in the WR plots.In spite of higher soil organic matter and microbialbiomass concentrations in the T2 treatment, there werelower concentrations of mineralizable N and plant Nhere than in the Tl treatment. Perennial grasses tendto retain N once they have taken it up, and may doso by creating and using a readily mineralized pool ofsoil N (Clark, 1977). Our data are consistent with thismechanism because treatments with the greatest Nmineralization in the laboratory also had the highestplant N concentrations in the field. Mineralization andplant N uptake were thus more closely related to each

Sample date

Treatment 3 August 7 September

gkg"Topsoil 1Topsoil 2Native

10.88.6

13.5

8.86.1

10.9HSD,'(0.05) = 1.7

than they were to microbial biomass. Microbial bio-mass concentrations were linearly related to soil or-ganic C and N concentrations, while respiration, Nmineralization, and plant N concentrations were in-fluenced by additional management factors.

In any ecosystem, the interaction of plant and mi-crobial processes constitutes nutrient cycling. What-ever regulates microbial processes also affects the plantcommunity. In this study, sustainable nutrient cyclesapparently required a minimum concentration of soilorganic matter between 1 and 7 g kg~' organic C. PlantN concentrations and plant growth were more relatedto mineralizable C and N than to soil OM or microbialbiomass concentrations. These data indicate that dis-turbed landscapes in this area may require special long-term management, such as annual fertilization, to in-sure persistence of a vegetative ground cover whensoil organic C levels are insufficient. A stable vegeta-tive community is necessary to prevent the loss of thesoil resource to erosion and to reestablish the produc-tivity of these lands.

ACKNOWLEDGMENTWe would like to thank Drs. E.T. Elliott, D.C. Coleman,

and J.F. Power and the journal reviewers for their helpfulreviews of the manuscript, and Dr. Elliott for his stimulatingdiscussions. We would also like to thank Chris Mahelonafor his able handling of the field and laboratory work.

MULLINS ET AL.: PLANT UPTAKE OF CADMIUM AND ZINC 1245