Effect of mild drying on the mineralization of soil nitrogen

Summary Drying soil to –100 kPa increased the subsequent mineralization of nitrogen under optimal moisture conditions. The effect was greater when the soils were dried to –1500 Pa. Mineralization was greater after four cycles of wetting and drying than after one. Depending on the drying conditions, the amount of nitrogen mineralized after drying to –1500 Pa was between 6.8 and 18.2% of that mineralized after chloroform fumigation. After drying the soils the average ratio of CO₂-C respired to min N was 21.1–22.3 depending on the drying conditions, whereas after chloroform treatment and autoclaving the ratio was 6.0 and 9.9 respectively. The effect of drying on nitrogen mineralization is attributed to two causes: the death and subsequent lysis of a small proportion of the soil organisms, and to the desorption of organic substances with a wide C/N ratio.Because of the stimulation of even mild drying conditions, marked differences in mineralization rates of soil nitrogen between cropping seasons must be expected.     

Production and nutrient dynamics of plant communities on a sub-Antarctic Island

Summary Studies of plant standing crop and of the nutrient concentrations in precipitation, soils and plants have enabled an assessment of the inter- and intra-system nutrient flows for five plant communities at Marion Island (46°54S, 37°45E). These communities, which are representative of those occupying more than 90% of the island's lowland (below 300m above sea level) were: a fjaeldmark on a rocky plateau (dominated by the cushion plant Azorella selago), an open fernbrake and closed fernbrake (both dominated by the fern Blechnum penna-marina) and two mire-grasslands (on very wet peats and dominated by graminoid and bryophyte species). Annual net primary production (ANP) at the five communities was high and substantial quantities of nutrients were taken up annually by the vegetation. N (6.5 to 24.8 g m^-2 year^-1) was the element taken up from the soil in the largest quantities, despite the fact that instantaneous values of available N pools were exceptionally low (0.003 to 0.69 g m^-2 to 25 cm depth). Either K (3.5 to 9.9 g m^-2 year^-1) or Ca (1.7 to 9.7 g m^-2 year^-1) was taken up in the second largest amount. Net quantities of nutrients translocated into the annual aboveground growth of vascular plants were, except for K and Na, greater than the seasonal mean standing stocks in the aerial biomass. Net translocation estimates ignored leaching losses from the biomass. Nutrient turnover times in the total (living plus dead, above- and belowground) vegetation were between 1 and 4 years, lower than for most Northern Hemisphere tundra communities. The quantities of nutrients in circulation were mostly less than 3% of their total pool (plants plus soil) sizes, except for K (13 to 26%) and, in four of the communities, Mg (6 to 15%). Precipitation inputs of N, K, Ca and Mg were considerably lower than the amounts required in the ANP. No P occurred in the precipitation. Biological fixation of N was much less than the precipitation input. The vascular plant species appear to be less efficient in conserving N through back-translocation from senescing photosynthetic tissue than are most plants of similar life forms from northern hemisphere tundra and tundra-like areas. Only 11 to 30% of the N taken up into the annual aboveground growth was back0translocated before or during senescence. Back-translocations of P (39 to 71%) and K (71 to 965) were greater. However, all of these estimates ignore leaching losses. Despite the apparently poor ability of the plants to back-translocate N, the total nutrient costs of the aboveground ANP at the five sites (14 to 32 mg per g m^-2 ANP) were very much in the lower part of the range reported for a wide variety of vegetation types. Nutrient costs of the ANP for the miregrassland communities were especially low, mainly because of low requirements for Ca and Mg. In view of the small soluble and available pools of some nutrients (especially N and P) and the substantial nutrient requirement for the ANP, it is concluded that net nutrient mineralization in decomposition and nutrient absorption by the vegetation are closely coupled.     

The influence of stress treatments on the microbial biomass and the rate of decomposition of humified matter in soils containing different amoutns of clay

Summary ¹⁴C-labelled substrates (glucose, hemicellulose, cellulose, maize straw, and barley straw) were incubated in 4 soils with clay contents of, 6, 12, 16 and 34%. After 2 years an average of 20% of the labelled C remained in the soils; 10% of this residual C was in biomass as determined by fumigation with CHCl₃.Air-drying, C addition (unlabelled glucose), heating (80°C), and grinding of the soils accelerated the evolution of labelled CO₂. Grinding and heating had the largest effect, increasing CO₂ evolution during the first 10 days by a factor of 15 to 22 relative to untreated soil. Air-drying had the least effect; it increased the CO₂ evolution 7 to 9 times. The accelerating effect was still measurable, during the third month of incubation when the CO₂ evolution was 1.2 to 1.9 times that from untreated soil.The treatments also affected the labelled biomass; air-drying had the least effect, and grinding the most. Three months after these two treatments the biomass was 3/4 and 1/4, respectively, of the amount at the start.On the average the treatments in all four soils had the greatest affect on humified material originating from glucose, hemicellulose, and cellulose; the least effect was on material originating from straw.The addition of unlabelled glucose accelerated the evolution of labelled CO₂–C in all four soils. The size of the effect on CO₂ evolution and on the biomass was similar to that of air-drying.Grinding killed a larger percentage of the biomass in the sandy soil than in the soils with a high content of clay. The effect of the other treatments was largely the same in all four soils.The effect of the treatments towards the native biomass and humic matter was largely parallel to that on the labelled biomass.The observations are consistent with the view that the biomass as determined by fumigation with CHCl₃ mainly consists of dormant organisms. CO₂ production — the biological activity — was related to the amount of available organic material and not the size of the biomass.     

Decomposition of organic matter and mineralization of nitrogen in brookston silt loam and alfalfa green manure

Summary C¹⁴ and N¹⁵ doubly labelled alfalfa tissue at addition rates of 5 and 1 ton per acre was incubated in the laboratory for 72 days with virgin and cultivated Brookston silt loam. The alfalfa tissue was more extensively decomposed in the virgin soils than in cultivated soil, but retention of tissue carbon was not affected by rate of addition. Percentage decomposition of organic matter in the virgin soil was greater than that in the cultivated soil. Addition of alfalfa tissue reduced decomposition of soil organic matter in proportion to the rate of addition and resulted in a gain of carbon in the incubation mixture. No priming action was noted.An increase in the rate of tissue addition caused an increase in the amount of nitrogen mineralized from the tissue but had little effect on the amount of nitrogen mineralized from the soil. Nitrogen mineralized from the soil organic matter was preferentially immobilized during the latter part of the incubation period.It appears that the organic matter content of the soil as well as the rate of tissue addition may regulate the priming action of green manures.Published with approval of the Director, Ohio Agricultural Experiment Station as Journal Paper No.94-62.     

Microbial contributions to the increased nitrogen mineralization after air-drying of soils

The amounts of mineral-nitrogen (NH₄−N+NO₃−N) extracted by 2MKCL and the net amounts of N mineralized (δ Min-N) during a 10-day incubation of field-moist soils, air-dried then rewetted samples, and chloroform-fumigated samples, were measured in a range of 20 topsoils from grasslands. Air-drying generally increased extractable-N and the δ Min-N of the remoistened soils, but decreased the Min-N flush after fumigation. The C∶N ratios (CO₂−C production: net Min-N production) over 10 days decreased significantly from an average of 25 to 12 after initial air-drying, suggesting that substrates of low C∶N ratio, such as microbial cells, were contributiong to the extra N mineralized after the air-drying treatment. A procedure to quantify the contribution from microbial-N to the increased δ Min-N after air-drying was only partially successful, but indicated a large proportion of this increase was derived from microbial cells killed by desccation.     

Determination ofin situ KN by the chloroform fumigation-incubation method and mineralization of biomass N under anaerobic conditions

A silt loam soil from Pakistan was incubated at 30°C with increasing levels (67, 133, 200, 267 and 333 μg N g^?1 soil) of¹⁵N-labelled (NH₄)₂SO₄ and glucose (C/N ratio of 30 for all additions). At a stage when all of the applied¹⁵N was immobilized (transformed into microbial biomass), moist soil samples were subjected to the chloroform fumigation-incubation method (CFIM) for determination of K_N and microbial biomass. Mineralization of biomass derived from the applied¹⁵N and the native soil N was studied under anaerobic conditions. In situ values of K_N varied from 0.19 to 0.42 and increased with increasing levels of amendment (N + glucose). From 10 to 18% of the native soil N was found as microbial biomass. Anaerobic incubation of the soils resulted in the mineralization (determined as NH ₄ ⁺ ) of 15.08 to 29.23% of the biomass¹⁵N at different levels of amendment; 2.90 to 4.43% of the native soil N was mineralized. From 70 to 90% of the N mineralized from native soil N was derived from microbial biomass; the rest was attributed to non-biomass N.     

Growth limitation of planktonic bacteria in a large mesotrophic lake

We studied nutrient-limitation of bacterioplankton growth in Lake Constance, a mesotrophic lake, between February and August in 1992. We amended 1-m filtrates with a single nutrient or nutrient combinations at 5 or 10 m final concentration, and the limiting nutrient or nutrient combination was inferred from the assay in which bacterial growth was most stimulated. The following nutrients were added individually or in combination: glucose, amino acids, peptone, and ammonium as C and N sources, and inorganic phosphate. From January until the beginning of the phytoplankton spring bloom in mid-April, C alone was growth-limiting. During the spring bloom a complex growth-limitation pattern occurred; first P was limiting, then for only 1 week C + N together, and thereafter P + C. During the clear-water phase with very low chlorophyll concentrations, P + C together limited bacterial growth again, interrupted by a period when C + N + P shortage caused a triple limitation. Later in the season, P + C were growth-limiting again. The growth efficiency (bacterial biomass produced/substrates used) on the basis of amino acid and carbohydrate used varied between 17 and 35%. The addition of various C and N sources indicated that the growth efficiency strongly depended on the quality of the substrates and the adaptation of the bacterial assemblages, for example, whether C and N originated from amino acids or glucose and ammonium.     

Chemistry and microbial activity of forest and pasture riparian-zone soils along three Pacific Northwest streams

Throughout the United States, agricultural practices are responsible for large quantities of nutrients entering lakes and streams. Previous studies have shown that forested riparian areas can filter nutrients from surface runoff and groundwater that may potentially contaminate lakes and streams. This study examined seasonal differences in soil chemistry and soil microorganisms in paired mixed-forest riparian and pasture systems, the aim being to gain understanding of the sequestering of N and P. The forest soils retained higher levels of organic C and N, mineralizable N, extractable P, and fungal biomass, and had higher respiration rates than pasture soils. These findings suggest that forested riparian zones have a greater capacity than pasture soils to sequester C and retain nutrients. In past studies, fungal biomass has been shown to be less than bacterial biomass in grassland soils, but in this study, fungal biomass was greater than bacterial biomass throughout the year in both forest and pasture soils.     

Changes in Cytoplasmic Carbon and Nitrogen Pools in a Soil Bacterium and a Fungus in Response to Salt Stress

The effects of water potential on the cellular compositions of a soil bacterium and a fungus were examined by growing the organisms in media with various KCl concentrations. In media containing up to 1 M KCl, C/N ratios in Aspergillus flavus increased significantly, while those for a Pseudomonas sp. did not. For both organisms, the proportions of cellular C and N in cytoplasm increased by a factor of 10 as salinity increased from 0 to 1 M KCl. Such compositional changes have implications for microbial biomass dynamics in soils of varying water potential and for biomass measurement by chloroform fumigation.     

Measurement of the microbial biomass in intact cores of soil

The fumigation/respiration technique was used to estimate the size of the soil microbial biomass. Sieving decreased the biomass in winter but increased it in summer; we suggest that this was a consequence of the different substrates available and the different microbial populations during the year. The flush in respiration following fumigation correlated significantly with the CO₂-C produced 10 days after fumigation (X), so that in the soils studied by us the biomass (B) can be calculated from Bk=0.673X–3.53, wherek is the fraction of fumigated organisms mineralized to CO₂, thus avoiding the need to measure CO₂ production from unfumigated cores.     

The kinetics of the degradation of chloroform and benzene in anaerobic sediment from the River Rhine

In anaerobic methanogenic sediment microcosms¹⁴C labelled chloroform was degraded mainly to carbon dioxide. At a concentration of 4 g.l^–1 the mineralization followed first order kinetics with a half life of 12 days at 10°C and 2.6 days at 20°C. At a concentration of 400 g.l^–1 the mineralization rate increased with time and followed logarithmic kinetics with a _max of 0.02.d^–1 at 10°C. The logarithmic kinetics can be explained by growth of the bacteria on the higher concentration of chloroform with a generation time of 35 days. Shaking and oxygenation did not inhibit the mineralization of chloroform, probably because of bacterial consumption of the dissolved oxygen. ¹⁴C labelled benzene was mineralized only for a small percentage to¹⁴C labelled carbon dioxide while other, not acid extractable, degradation products were formed. Under anaerobic conditions after one day when 5% of the benzene was degraded to carbon dioxide, the mineralization ceased, while the disappearance of benzene proceeded. With air in the headspace of the incubation bottles 25% of the benzene was mineralized to carbon dioxide. The anaerobic degradation of benzene at a concentration of 100 .l^–1 showed similar kinetics as the degradation at 1 g.l^–1. Hence no adaptation of the microflora in the sediment occurred during the 63 days of the experiment at 100 g.l^–1.     

Dynamics of the internal soil nitrogen cycles under moder and mull forest floor types on a slope in a <Emphasis Type="Italic">Cryptomeria japonica</Emphasis> D. Don plantation

To examine the influence of microbial carbon (C) availability on the internal soil nitrogen (N) cycles under moder and mull forest floor types within the same slope sequence, surface mineral soils (0–5cm depth) taken at upper (moder-type forest floor) and lower (mull-type forest floor) positions on a slope in a Cryptomeria japonica D. Don plantation were incubated for 300days. During the incubation, changes in net and gross N transformations, the organic C and N pools, and microbial respiration were monitored. Despite relatively small differences in net N mineralization in both soils, very rapid rates of gross N transformations were found in mull soil during the initial 15days of the experiment. A rapid net nitrification occurred after days 150 and 100 in moder and mull soils, respectively, presumably because of decreased microbial C availability. However, a rapid net nitrification also occurred in the mull soil during the initial 15days when microbial C availability was high, and gross nitrification was detected in both soils, except at day 0 in the moder soil. Changes in gross N transformations and in organic C and N pools over the experiment suggested that the start of rapid net nitrification might be influenced not only by microbial C availability, but also by the microbial availability of N relative to C.     

Long-term effects of elevated nitrogen on forest soil organic matter stability

Nitrogen addition may alter the decomposition rate for different organic-matter pools in contrasting ways. Using a paired-plot design, we sought to determine the effects of long-term elevated N on the stability of five organic-matter pools: organic horizons (Oe+a), whole mineral soil (WS), mineral soil fractions including the light fraction (LF), heavy fraction (HF), and a physically recombined fraction (RF). These substrates were incubated for 300 days, and respiration, mineralized N, and active microbial biomass were measured. Samples with elevated N gave 15% lower cumulative respiration for all five substrates. Over the 300-day incubation, the Oe+a gave twice the cumulative respiration (gCkg^–1 initial C) as the LF, which gave slightly higher respiration than the HF. Respiration was 35% higher for the WS than for the RF. Mineralized N was similar between N treatments and between the LF and HF. Net N mineralized by the LF over the course of the 300-day incubation decreased with higher C:N ratio, due presumably to N immobilization to meet metabolic demands. The pattern was opposite for HF, however, which could be explained by a release of N in excess of metabolic demands due to recalcitrance of the HF organic matter. Mineralized N increased with respiration for the HF but showed no pattern, or perhaps even decreased, for the LF. WS and RF showed decreasing active microbial biomass near the end of the incubation, which corresponded with decreasing respiration and increasing nitrate. Our results show that long-term elevated N stabilized organic matter in whole soil and soil fractions.     

Geoecological alteration of surface soils by the Hawaiiansilversword (Argyroxiphium sandwicense DC.) inHaleakala's crater,Maui

The geoecological effects of organic matter addition to soil byHawaiian silverswords (Argyroxiphium sandwicense DC.,Asteraceae) in Haleakala's crater (Maui) are examined. Sets of 12 surfacesoil samples were gathered in three positions: bare soils; under the crowncanopy of adjacent live plants; and below dead, withered rosettes. Silverswordstanding litter and live foliage were also collected. Several physical andchemical properties were investigated. Bare soils were coarse and exceptionallylow in chemical nutrients and water-holding capacity. Litter addition belowsilverswords altered all properties except particle-size distribution; soilsunder dead rosettes became modified more strongly than those below live plants.While rosettes grow, standing litter is tightly preserved, thus only smallamounts of organic matter are added to soil. When plants die, a substantialinflux of litter is released in a short period (7 to 9 years average); thisraises the C and N content, C/N ratios, exchangeable nutrient levels (Ca, Mg,K, P) and cation exchange capacity of soil. Physical properties, such as color,bulk density and porosity are also greatly affected; as a result, soilwater-retention at field capacity nearly doubles under withering plants. Soiltemperatures (5 cm depth) at noon were 18.1°C lower beneath silversword canopies(18.5°C mean) than in contiguous bare soils(36.6°C). This cooling may result in lower evaporationand further conservation of soil moisture, thus increased seedgermination. Nutrient-retention strategies may have evolved in the semelparousArgyroxiphium to help it cope with the harsh,drought-prone soils and unpredictable climate of Haleakala. Silverswordflowering results in a tremendous seed output, but the plant needs to ensurethat some seeds – which have limited dispersal capability – willgerminate, and thus provides a nutrient-rich, water-retaining substrate moresuitable for seedling establishment than bare soils. Through itsself-regulating influence on underlying soils,Argyroxiphium affects both soil formation and its ownregeneration in Haleakala.     

Fine root biomass and tree species effects on potential N mineralization in afforested sodic soils

The study was carried out under three types of plantation forest of 40 years, growing on infertile sodic soils, poor in organic matter and N content, of Indogangetic alluvium at Lucknow (26°45 N; 80°53 E). Fine root biomass estimated under three forests did not differ much with season, or with species (106–113 g m^-2) but varied with soil depth to 0.45 m. The proportion of very fine roots (<0.5 mm) increased with soil depth. Available N in soil was greatest under mixed forest followed by Eucalyptus camaldulensis and Acacia nilotica planted soils. N was maximum in summer season and decreased with soil depth. Nitrogen mineralization during anerobic incubation of 14 days could not be differentiated by tree species, but the monsoon season favoured the process and winter season retarded it. Mineralization decreased with soil depth corresponding to fine roots. There was a reduction in bulk density of soil, pH and EC in forested soil compared to a similar but non forested soil, whereas, organic C and total N increased in forested soils. N mineralization was found to be affected significantly with the fine root biomass and available N content in the soils, whereas negative relations of mineralized N with pH and EC were noticed, though these were not significantly different in this study.     

Soil lipase activity – a useful indicator of oil biodegradation

The evaluation of soil lipase activity as a tool to monitor the decontamination of a freshly oil-polluted soil was tested in a laboratory study. An arable soil was experimentally contaminated with diesel oil at 5 mg hydrocarbons g^–1 soil dry weight and incubated with and without fertilization (N-P-K) for 116 days at 20°C. Lipase activity and counts of oil-degrading microorganisms were measured at regular time intervals, and the correlations with the levels of hydrocarbon concentrations in soil were investigated. The residual soil hydrocarbon concentration correlated significantly negatively with soil lipase activity and with the number of oil-degrading microorganisms, independent of fertilization. The induction of soil lipase activity is a valuable indicator of oil biodegradation in naturally attenuated (unfertilized) and bioremediated (fertilized) soils.     

Cytotoxic activity and T cell receptor repertoire in tumor-infiltrating lymphocytes of adrenal cell carcinomas

The cytotoxic activity and T cell receptor (TCR) V repertoire in tumor-infiltrating lymphocytes (TIL) of three primary adrenal cell carcinomas were analyzed. Fresh, non-cultured TIL from two of the three tumors showed low but significant lysis of the autologous tumor, and for one of the patients this activity was strongly enhanced upon culture in interleukin-2. An allogeneic adrenal cell carcinoma line and the K562 or Daudi targets included as controls were not killed. Phenotypic analysis of freshly isolated TIL demonstrated that the cells from the two patients that demonstrated cytolytic capacity mainly consisted of CD45RO⁺ T cells. In vitro cultured TIL lines from these patients demonstrated a high percentage of CD8⁺ cells expressing either the V6 gene or the V8 gene product, as measured with a panel of mAb specific for TCR V and V gene products. Analysis of the TCR V gene mRNA expression in freshly isolated non-cultured TIL, using a polymerase-chain-reaction-assisted cDNA-amplification assay, confirmed the strong expression of the genes coding for the TCR V6 or the V8. This assay also demonstrated a more restricted TCR V gene usage in the TIL as compared to peripheral blood lymphocytes from the same patient.This study was supported by the Swedish Cancer Society and by the Cancer Society in Stockholm     

Gross nitrogen transformations in soils from uncut and cut boreal upland and peatland coniferous forest stands

Gross and net nitrogen (N) ammonification and nitrification were measured in soils from an uncut and recently cut upland and peatland conifer stand in northwestern Ontario, Canada. Rates of gross total inorganic N immobilization were similar to gross mineralization, resulting in low net mineralization rates in soils from all four upland and peatland conifer stands. Gross ammonification rates were variable but similar in soils from uncut and cut peatland hollows (18–19mgNkg^–1day^–1) and upland forest floor soils (14–19mgNkg^–1day^–1). Gross ammonium ( ) immobilization rates were also variable but similar to ammonification rates. Median gross nitrification rates were within 0–2mgNkg^–1day^–1 in soils from all four upland and peatland cut and uncut stands, although rates were consistently higher for the soils from the cut stands. Large variability in gross nitrification rates were observed in peatland soils, however the highest gross nitrification rates were measured in saturated peatland soils. Net rates remained low in the soils from all four stands due to high nitrate ( ) immobilization and very fast turnover (<0.2 day). Our results suggest that potential losses may be negated by high immobilization in uncut and cut boreal forest stands. This study reveals the potential for the interaction of N production and consumption processes in regulating N retention in upland and peatland conifer forests, and the resilience of the boreal forest to disturbance.     

The growth of fractal humic acids: Cluster correlation and gel formation

The growth of humic acids, prepared by a gentle method from two different kinds of soils (I and II), has been studied using small-angle neutron scattering at an acidity corresponding to pH 5.0 and 0.10 M ionic strength (NaCl). Humic acids aggregate either to large clusters with a fractal dimension of 2.3₅ and an average diameter of 1720 (I) or to clusters with an average diameter of 700 (II). After storage for 2 days at 4 °C, the latter aggregates (II) formed a gel. In a step toward gelation, we observed cluster-cluster interaction from the neutron-scattering data in the form of a correlation peak. These differences in size can be explained by assuming that the smaller particles (II) are trapped into a nonequilibrium state characterized by the temperature-solvent condition. The importance of a humic acid gel network for the conservation of water and nutrients in the environment is discussed.     

Soil nitrogen dynamics along a gradient of long-term soil development in a Hawaiian wet montane rainforest

I analyzed the rates of net N mineralization and nitrification of soils from seven sites in a Hawaiian wet montane forest. The sites differ in age, ranging from 400 to 4,100,000 yr, but are comparable in other variables (all at 1200 miasl with 4000 mm or more mean annual rainfall), and the chronosequence simulated a development of soils from basaltic lava. Soils were incubated for 20 days at 17.5 °C, which is nearly equivalent to a mean field air temperature of the sites, and at an elevated temperature of 25.5 °C under three treatments: 1) field-wet without amendments, 2) air dried to a permanent wilting point, and 3) fertilized with phosphate (NaH₂PO₄) at the rate of 50 g P per g dry soil. Both mineralization and nitrification rates varied significantly among the sites at the field temperature (p<.00001). Fractions of the mineralized organic matter (indexed by the N produced per g organic C) increased sharply from the youngest to the 5000-yr site before declining abruptly to a near constant value from the 9000 to the 1,400,000-yr sites. Total organic C in the top soils (<15 cm deep) increased almost linearly with age across the sites. Consequently, net NH₄- and NO₃-N produced on an area basis (g m^-2 20 d^-1) increased sharply from 0.2 in the youngest site to 1.2 in the 5000-yr site, then both became depressed once but steadily increased again. The fraction of organic matter mineralized, and the net N turnover rates were outstandingly high in the oldest site where a large amount of organic matter was observed; the topsoil organic matter which was used in this analysis appeared to be highly labile, whereas the subsurface organic matter could be relatively recalcitrant. As suggested by earlier workers, the initial increase in N turnover seemed to correspond to the increasing quantity of N in the soils through atmospheric deposition and biological fixation. The later decline in fraction of organic matter mineralized seemed to relate to increasing soil C/N ratios, increasingly recalcitrant organic matter, and poorer soil drainage with age. The elevated temperature treatment produced significantly higher amounts of N mineralization, except for the youngest site where N was most limiting, and for two sites where soil waterlogging might be severe. P fertilization invariably resulted in slower N turnovers, suggesting that soil microbes responded to added P causing N immobilization. The youngest site did not significantly respond to added P. The magnitude of immobilization was higher in older than in younger soils, suggesting that P more strongly limits microbial populations in the older soils.