assessing soil c sequestration and nutrient cycling under ...kearney.ucdavis.edu/old...

2001-2006 Mission Kearney Foundation of Soil Science: Soil Carbon and California's Terrestrial Ecosystems

Final Report: 2005226, 1/1/2006-12/31/2006

1University of California, Davis, Land Air and Water Resources (LAWR) 2University of Wyoming, Renewable Resources Department

Assessing Soil C Sequestration and Nutrient Cycling Under Secondary Wastewater Irrigated Poplar Plantations William R. Horwath1* and Urszula Norton2

Summary Treated wastewater disposal is a mounting problem faced by treatment facilities across California and the United States. This issue is particularly acute in rural, mountainous, and rapidly urbanizing regions like the Sierra Foothills and other rural communities in which improvements in infrastructure are difficult to maintain. Treated municipal wastewater has been shown to be effective to grow high biomass, which is then used to provide electricity for wastewater treatment facility operations (Stanton et al. 2002). The application of treated wastewater creates potential for significant C storage in woody biomass, but also, annual surface litter deposition and root turnover provides additional benefits of enhanced soil C sequestration. In addition, potential C and N losses to greenhouse gas emissions (GHG) may also be potentially reduced through the formation of stable soil organic matter (SOM). The application of wastewater has been studied mainly from its silvicultural effectiveness to promote biomass production while relatively little research has focused on belowground C sequestration. This research generated much needed information on the process of belowground C sequestration and GHG emissions from wastewater treated biomass plantations. We hypothesized that treated wastewater irrigation would optimize poplar biomass production and lead to SOM formation, reducing the potential to losses of C and N to greenhouse gases. Our results suggest that converting wastewater-irrigated pasture to poplar stand resulted in changes in belowground C and N pools. We observed a decline in DON, DOC, microbial biomass and potentially mineralizable N (PMN) concentrations in the upper 5 cm of soil. Older poplar stands accumulated more DOC with time in the subsurface soil horizons (5-15 and 15-30 cm) compared to younger poplar stands and pastures. Soils beneath short-term rotation poplar vegetation irrigated with wastewater are not effective in C and N storage and GHG mitigation. With progressing stand age soils beneath poplar vegetation become gradually N saturated, resulting in higher litter N content, litter decomposability, and greater N2O flux to the atmosphere compared to pasturelands.

Objectives The purpose of this project was to inventory the effects of secondary wastewater inputs on belowground C sequestration and nutrient cycling across a wide spectrum of poplar plantations and other systems capable of accepting treated water.

Objective 1: Assess the effects of secondary wastewater irrigation on belowground C and N sequestration and mineralization.

Objective 2: Evaluate the effects of seasonal wastewater additions on GHG emissions.

Assessing Soil C Sequestration and Nutrient Cycling Under Secondary Wastewater Irrigated Poplar Plantations—Horwath

2

Figure 1. Soil total N and total organic C for 0- to 30-cm depth.

Approach and Procedures

Location A 15-acre poplar plantation and nearby pasture owned and managed by the Jamestown Sanitation District, which is adjacent to and irrigated from the Tuolumne Utility District treated wastewater storage reservoir. Trees have been planted each year since 1999, resulting in seven age cohorts.

Soil Sandy clay loam about 4-feet deep (Ag West Resources, 1997). Wastewater application: The drip irrigation system was designed to meet the peak estimated transpiration rates of 0.26 inches


3

per day. The average maximum daily temperature is 95.2ºF, and poplars can transpire an estimated 43 inches of water over a nearly 250-day growing season (personal communication, Dr. Larry Schwankl, UC Davis) (Braatne 1999).

Figure 2. Soil available C and microbial biomass C for 0- to 5-cm, 5- to 15-cm, and 15- to 30-cm depths.

Field sampling design The following sites (total of nine) were identified for this research: Name Vegetation Irrigation treatment Objective 1 Objective 2CONTROL grassGF grassGW grassPW2 poplar 2-year old wastewater irrigated (3 yrs)PW3 poplar 3-year old wastewater irrigated (4 yrs)PW4 poplar 4-year old wastewater irrigated (5 yrs)PW5 poplar 5-year old wastewater irrigated (6 yrs)PW7 poplar 7-year old wastewater irrigated (8 yrs)PF7 poplar 7-year old ditch water irrigated

ditch water irrigated1-year wastewater irrigated

no water added


4

Sampling (Objective 1) Six randomly sampled soil cores were collected from each transect within each site in the spring before wastewater application started. Cores were stratified by depth and separated to 0-5 cm, 5-10, and 15-30 cm. Samples from each transect were bulked and homogenized in the field, all visible coarse fragments (greater than 4 mm) removed, and subsamples drawn for field extraction in 0.5 M K2SO4. The remainder of the composite samples were bagged and stored on ice for transportation to the lab and further analyses. Additional intact soil cores were excavated for estimates of soil bulk density.

Figure 3. Litter biomass, litter total N, litter total organic C and litter decomposability and poplar stand age.

Laboratory analyses

Soil samples were analyzed for: particle-size distribution by hydrometer (Gee and Bauder 1986); bulk density by volume and mass (Blake and Hartge 1986); inorganic C by gravimetric determination (Staff 1954); total C and N by Carlo Erba combustion (Dumas 1981); field-extracted mineral N; mineralizable C and N by aerobic incubation-extraction (Hart et al. 1994, Zibilske 1994); dissolved organic C and N; microbial biomass C and N by fumigation-extraction (Horwath and Paul 1994); and potentially mineralizable N (PMN).


5

Sampling (Objective 2) Trace gas monitoring was performed weekly during the growing season (17 weeks). An enclosure technique for measuring CO2, CH4, and N2O fluxes were used for this experiment (Standard Soil Methods for Long-Term Ecological Research, page 188). Trace gas flux measurements were taken using static chambers deployed on the soil surface for a period of 30 minutes (Hutchinson and Mosier 1981). Soil water content was monitored daily using TDR probe. Soil samples were taken at the same time GHG measurements were performed.

Figure 4. Soil nutrient pools for 0- to 5-cm, 5- to 15-cm, and 15- to 30-cm depths.

Laboratory analyses

Gas samples were analyzed for using Automated Gas Chromatograph (Varian 38001) equipped with thermoconductivity, flame ionization and electron capture detectors to capture CO2, CH4, and N2O, respectively (Mosier and Mack 1980). Best fluxes were estimated from the rate of


6

change of the gas concentration in the chamber headspace. Soil 0.5M K2SO4 extracts were analyzed for inorganic N, DOC and DON.

Figure 5. Correlation between soil DOC and soil water content.

Results

Soil profile nutrient pools Our results suggest significant differences in soil profile TOC and TN between treatments (fig. 1). Irrigation applied to grass vegetation increased soil profile TN by 62% and TOC by 45% compared to control. Poplar stands showed TN and TOC losses in soil profiles with the lowest levels reported for PW3. Soils beneath PW4 and older had TN and TOC higher than PW2. The greatest TN and TOC contents, comparable to those determined to GW and GF, were observed in soils beneath PW5. Soils beneath PW7 had 69% more TN and 40% more TOC compared to the same age stand irrigated with ditch water (PF7). Soil TN and TOC distribution within 0-30 cm soil profile showed an average of 3% TN and TOC loss from 0-5 cm and 2% gain in 15-30 cm per each year of the poplar stand age (table 1).

There was a significant increase in available C in 0-5 cm and 5-15 cm soils between control and GF and GW (fig. 2a). Available C in 0- to 5-cm soil beneath poplar vegetation, regardless of age, was considerably lower compared to grass vegetation. The lowest available C in 0- to 5-cm soil was in PW4 soils and the highest in PW5 soils. Soils beneath PW7 and PF7 had similar levels of available C in 0-5 cm that averaged 1284 mg CO2 C-1 g-1 21 d-1. The highest of all soils available C in 15-30 cm was reported for PW5 and PW7. Soils beneath PW7 had 56% more available C in 15-30 cm than PF7. With progressing age of polar stands, a loss of 234.3 mg g-1 21 d-1 per year from 0- to 5-cm soil and a gain of 22.4 mg g-1 21 d-1 in 15- to 30-cm soil occurred (table 1).

Soils beneath grass vegetation had significantly greater microbial biomass C concentrations in 0- to 5-cm soil compared to soils beneath poplar vegetation (fig. 2b). The highest of all


7

microbial biomass C concentrations in 0- to 5-cm and 5- to 15-cm soils were in GF soils and the lowest concentrations were in PW2 and PW4 soils. There was a loss of 18.5 ug g-1 of microbial biomass C from 0- to 5-cm soil per each year of poplar stand age (table 2). No relationship between stand age and microbial biomass C concentrations was determined for other soil layers.


8

Figure 6. CO2, N2O, and CH4 production.

There was a strong relationship between stand age and surface litter quality and quantity (fig. 3). Accumulation of litter biomass increased from 0.3 T ha-1 in PW2 to 0.8 T ha-1 in PW7 and the average rate of litter increase was 0.1 t ha-1 per each year of poplar age (fig. 3a). Soils beneath GW and PF7 had on average 0.2 T ha-1 of biomass litter and control had 0.06 T ha-1. Litter TN increased with stand age from 0.003 T ha-1 in GW to 0.01 T ha-1 in PW7 at the same time litter TN in control averaged 0.001 T ha-1 and in PF7 litter TN was 0.002 T ha-1 (figs. 3b and 3c). The average rate of TN increase was 0.002 T ha-1 per each year of poplar age. Similarly, the older the stand, the more litter TOC we observed. The TOC values ranged between 0.06 T ha-1 in GW to 0.41 T ha-1 in PW7. Control averaged 0.05 T ha-1 and PF7 averaged 0.07 T ha-1. The rate of litter TOC accrual was 0.04 T ha-1 each year of stand age. Interestingly, litter decomposability as represented by the ratio of lignin to TN also increased with stand age (fig. 3d). The ratio declined from 30:1 determined for GW to 14:1 determined for PW7 at an average rate of 3.4 per each year of stand age (fig. 3d). The only exception was PW5 where the ratio of lignin to TN was the lowest of all and averaged 4:1. Litter in control had ratio of lignin to TN of 25:1 and PF7 had an index of 31:1.

Soil nutrient concentrations showed significant differences between treatments (fig. 4). All poplar stands irrigated with treated wastewater had lower NH4 levels compared to grass vegetation and PF. The highest NH4 concentration in 0-5 cm soil was in GW (7.6 ug g-1) and the lowest was in PW3 (1.7 ug g-1). Soils beneath PW5 had the greatest increase in NH4 concentration in the bottom 15-30 cm of soil and soils beneath PW7 had the greatest increase in 5-15 cm of soil. Soils beneath PF7 had the most uniform NH4 distribution throughout the soil profile, averaging 4.0 ug g-1.

There was a significant increase in NO3 concentrations in all soils irrigated with wastewater in 0- to 5-cm layer and poplar soils only in 5- to 15-cm and 15- to 30-cm layers (fig. 4). The highest NO3 concentration in 0-5 cm was in PW2 (9.7 ug g-1) and the lowest NO3 concentrations were beneath PW3 and PW4 (4.5 and 4.8 ug g-1, respectively). Soil NO3 in 0- to 5-cm soil beneath PW5 and PW7 was on average, 1.7 times greater than beneath PW3 and PW4. Most poplar soils had on average NO3 concentrations of 1.52 ug g-1 in 15-30 cm soil layer, except for PW5 where it was slightly higher and averaged 2.2 ug g-1.

When comparing PMN estimates, our results showed higher concentrations in 0-5 cm in soils beneath irrigated grass compared to poplar vegetation (fig. 4). The highest PMN levels of all were in GF (97.6 ug g-1 14 d-1), followed by GW (77.8 ug g-1 14 d-1), and control (49.9 ug g-1 14 d-1). Most poplar soils had comparable PMN concentrations in 0- to 5-cm soil that averaged 30.0 ug g-1 14 d-1, except for PW7 where the PMN was slightly higher (34.4 ug g-1 14 d-1). Soils beneath grass vegetation and PF7 had higher PMN concentrations in 5- to 15-cm layer compared to wastewater irrigated poplars. The highest PMN in 15- to 30-cm soil was in control.

The highest DOC in 0- to 5-cm soil was found beneath irrigated grass vegetation where it averaged 92 ug g-1 for both GF and GW (fig. 4). Most poplar soils had comparable DOC concentrations in 0-5 cm that averaged 51 ug g-1 except for PW2 where it equaled 71 ug g-1. There was a significant increase in DOC in 15-30 cm in soils beneath PW5 and in 5-15 cm in PW7.


9

Soils beneath irrigated grass vegetation had higher DON concentrations at 0-5 cm compared to other soils (fig. 4). The greatest DON levels were in GW soils (17.9 ug g-1) followed by GF (12.7 ug g-1). Soils beneath PW2 and PW5 had similar DON levels that averaged 9.3 ug g-1 and were the highest of all poplar soils. Soil DON beneath PW3, PW4 and PF7 were low and similar to each averaging 5.7 ug g-1.

Figure 7. Litter biomass and litter total N change during the growing season.

Summer soil nutrient pools and GHG emissions Summer average air temperatures were high and averaged 30.1ºC for all treatments (table 2). Soil temperatures were also high and the average temperature for irrigated plots was 23.1ºC which was 6.8º lower than soil temperatures in control. Soil water content averaged 0.16 g g-1 for all wastewater irrigated plots except for PW5 where it was 0.20 g g-1. Control had the lowest of all soil water content that averaged 0.02 g g-1. Soils in control plots also had the highest percentage of sand, bulk density, NH4, DON and DOC concentrations, and the lowest percentage of silt, clay and pH, compared to other soils. Soils beneath GW had the lowest percentage of clay and NO3, and the second highest NH4, DOC and DON concentrations. Soils beneath poplar vegetation had on average more silt and clay and less sand compared to soils beneath grass vegetation. Furthermore, these soils also had lower NH4, DON, and DOC concentrations. Soils beneath PW2 and PW7 had NO3 and NH4 concentrations significantly greater than PW3 and PW5. The lowest NO3 of all poplar soils was in PW3 soils. This soil also had the lowest NH4 and


10

DON concentrations of all soils. The lowest DOC concentrations were in PW7. Soils beneath PW2 had the lowest and beneath PW5 the highest pH of all soils.

Table 1. Correlation between stand age and soil profile total N, total organic C, available C and microbial biomass C.

Table 2. Air temperature, soil temperature, soil water content, soil texture, bulk density and soil nutrient concentrations measured over the course of irrigation season.

Soil DOC was well correlated with soil water content during the irrigation season (fig. 5).

High soil DOC levels were in dry soils (below 0.03 g g-1) and the wetter the soil the lower DOC levels.

The highest average CO2 flux was reported from soils beneath GW (2992 kg ha-1 season-1) (fig.6a) and the lowest from control (281 kg ha-1 season-1). In poplar soils, the lowest CO2 flux was from soils beneath PW2 and the highest CO2 flux was from PW3. When excluding PW3 soils, CO2 flux increased with stand age at a rate of 3.9 mg m-2 hr-1 per each year of stand age (Rsq=0.6, n=15) (data not presented). Soil CO2 flux increased at a rate of 47.4 mg m-2 hr-1 with increasing soil TOC content (table 3).


11

All soils assimilated CH4 during the irrigation season (fig. b). The lowest average seasonal assimilation occurred in PW7 while the greatest soil assimilation was in control (-0.1 kg ha-1 season-1). Soils beneath poplar vegetation decreased CH4 assimilation as poplars grew older, which occurred at a rate 0.74 ug m-2 hr-1 per each year of stand age (table 3). In addition, CH4 assimilation declined with decreasing litter decomposability at a rate -0.1 ug m-2 hr-1 (table 3). Table 3. Correlation between CO2, CH4, N2O and soil total organic C, stand age, litter total N and lignin:N.

Control soils produced the lowest amounts of N2O (0.03 kg ha-1 season-1) and PW5 produced

the greatest amount (5.9 kg ha-1 season-1) (fig. 6c). The second highest N2O production occurred in soils beneath PW7. Excluding PW5, all other poplar soils showed a linear N2O increase with stand age at a rate 43.2 ug m-2 hr-1 per each year of poplar age (table 3). In addition, N2O production increased with increased litter TN content at a rate 43.2 ug m-2 hr-1 per unit TN and declined with declining litter decomposability at a rate -2.1 ug m-2 hr-1 per unit lignin/TN (table 3). Estimates of GWP also increased with declining litter decomposability at a rate of 35 meq CO2-C ha-1 yr-1.

Seasonal litter deposition during the irrigation season was significantly greater in all poplar soils compared to control and GW (fig. 7). The greatest litter biomass TN was in PW5. Soils beneath GW showed a net loss in biomass that occurred in the course of the irrigating season.

Discussion Our results suggest significant differences in C and N pool sizes and pool distribution in soil profile between wastewater irrigated grass and poplar vegetation. Shift in life form from grass to deciduous tree vegetation and associated change in plant-derived organic matter, resulted in a notable decline in soil TN and TOC in 0–30 cm depths and accumulation of TN and TOC in surface litter biomass. This transition was already visible in the youngest poplar stands. Changes


12

in soil C and N distribution was also demonstrated as a shift in TN and TOC distribution within the soil profile. We observed a loss of 3% of soil profile TN and TOC from 0- to 5-cm soil each year the poplars grew older. Part of soil profile TN and TOC (2%) was relocated to the 15-30 cm of soil and part was deposited on the surface in litter.

One of the possible causes of surface soil TN and TOC losses was enhanced leaching of DOC and available C to the lower soil horizons. This resulted in the presence of available C in quantities insufficient to support microbial biomass growth and activity in 0- to 5-cm soil. Less microbially available C, low microbial biomass C concentrations, and higher levels of NO3 in the 0- to 5-cm layer also suggested less microbial N immobilization and more N mineralization in poplar soils. Available C and N translocation to lower soil horizons can further exacerbate soil TOC and TN loss from poplar soils through enhanced mineralization and priming of protected soil organic matter.

Another cause of soil TN and TOC loss is by interception in the surface-accumulated plant litter. Poplar litter contains approximately 80% leaf residues and can provide an important form of C and N storage (Bernal et al. 2003). Our results suggest, however, that the TN and TOC build up in litter biomass is very slow at 0.002 T ha-1 TN and 0.04 T ha-1 TOC per each year of poplar age. The TN and TOC gains are also proportionally smaller compared to gain in litter biomass over the growing season.

Interestingly, litter decomposability also increased with the age of poplar stand. Bernal et al. (2003) estimated leaf litter decomposition of alder and poplar leaves at three times higher than other deciduous species. High litter decomposability is a function of continuous supplies of readily available N delivered to the soil with wastewater irrigation. Poplar litter is deposited in a pattern of layers of different degree of decomposition (data not presented). The surface layer is most exposed to external environmental conditions such as extreme drying, rewetting and wastewater application while the bottom layers are more protected and can more efficiently store available nutrients and water as demonstrated by the 0.01 to 0.04 g g-1 soil water content increase in the older poplar stands. Gradual accumulation of surface litter suggested that despite an increasing litter decomposability with progressing poplar stand age, litter deposition can surpass litter decomposition.

Soils beneath poplar vegetation became bigger sources of GHG emissions with stand age. Not only N2O and CO2 production increased as stands grew older, but also CH4 assimilation declined with time. Gaseous N emissions are thought to be a major pathway of N loss from wastewater-irrigated systems. The soil processes that result in N2O production are associated with denitrification or nitrification (Mosier et al. 1997). Denitrification generates N2O when soils are saturated, which happened during the irrigation or shortly after, when water availability was high and soil temperatures averaged 22ºC creating optimal environment for pulses of microbial activity and spontaneous volatilization. To avoid the effects of water pulse on the effects of wastewater irrigation on soil C, N and GHG fluxes, we performed our measurements 24 hours after irrigation shut off.

Interestingly, N2O production did not correlate well with soil TN, but instead increased with increasing litter TN and decomposability. Thus we propose that significant amounts of labile N delivered to soil surface with wastewater irrigation were lost from the litter-soil continuum to volatilization and GHG emissions before they entered the soil.


13

Highly decomposable N rich litter can also become a source of N2O generated by denitrifcation during irrigation or nitrification during the dry down (Menyailo and Hungate 2006). Correlation of N2O production with soil C abundance has been observed in a wide scope of field and laboratory measurements. For example, Ambus and Christensen (1995) observed that dissolved organic carbon, a direct source of energy for denitrifiers, is one of the major limiting factors for N2O production. During our experiment we did not find a strong correlation between soil DOC concentrations and N2O production.

Methane present in the atmosphere is removed from the air as a result of biological oxidation conducted by soil microbes under aerobic conditions. Factors affecting CH4 assimilation include soil NH4

+-N concentrations (high enzymatic similarity and preferential substrate use) and soil water content (Mosier et al. 1991). In our study, CH4 assimilation declined with stand age and as litter decomposability decreased, suggesting that excess irrigation-applied wastewater surpassed soil capacity to assimilate all CH4.

Weekly monitoring during the summer irrigation season revealed important differences in management, maintenance and history between poplar stands. Some of these differences became critical in our data interpretation.

For example, PW2 was densely planted into the previously grass vegetated site that was under wastewater irrigation for one year. Thus soil nutrient pools and patterns of GHG emissions reflected in part the origin of organic matter in the soil previously associated with grass vegetation. Our results suggested that soils beneath PW2 experienced increased mineralization of soil C and N previously accrued under grass vegetation. It was demonstrated by high soil NO3 and DOC concentrations in 0- to 5-cm soil and low microbial biomass C and available C content in the spring. Furthermore, CO2 and N2O were the lowest and soil NO3 and DOC concentrations were the highest of all poplar soils during the irrigation season, which could suggest suppressed microbial activity and excessive amounts of wastewater application to PW2. The highest CH4 assimilation of all poplar soils in conjunction with low PMN and available C in 0- to 5-cm soil compared to GW, suggested that soil biochemical processes in PW2 are still in transition between grass and poplar vegetation.

As trees turned 3 years old, a thinning practice in early spring was performed in PW3, resulting in minimal soil disturbance, large openings between trees and enhanced grass growth utilizing available light during summer. Presence of grass vegetation and high litter TOC content could explain the highest CO2 production of all poplar soils.

There was a long-term irrigation piping failure in PW5 detected in the middle of the summer. The hole in the pipe resulted in excessive amounts of wastewater that had been delivered to few locations in PW5 for a couple of years and resulting in surface water ponding that often lasted longer than 12 hours after irrigation shut off. High soil profile TN and TOC contents, high litter decomposability, high average soil water content and excessively high N2O fluxes could be explained by increased amounts of wastewater applied to PW5. Interestingly, excessive wastewater application did not affect CH4 assimilation or CO2 production, thus supporting the previous conclusion of a significant portion of N2O generated in the litter or at the litter-soil interface.

One year wastewater irrigation of grass vegetation had a significant impact on GHG emissions and available soil nutrient pools without any affect on soil profile TOC and TN


14

storage in GW. High soil NH4, NO3, DOC, DON and available C concentrations in 0-5 cm and lower microbial biomass C and PMN in spring in GW compared to GF soils suggest presence of labile C and N in excess of the plant demand and soil ability to fix. The highest CO2 production of all soils, high N2O emissions and low CH4 assimilation compared to control soils indicated enhanced mineralization and rapid loss of C and N from the soil.

In summary, contrary to our expectations soils beneath short-term rotation poplar vegetation irrigated with wastewater are not effective in C and N storage and GHG mitigation. With progressing stand age soils beneath poplar vegetation become gradually N saturated, resulting in higher litter N content, litter decomposability, and greater N2O flux to the atmosphere compared to pasturelands.

References Ag West Resources. 1997. Beneficial agricultural reuse of treated municipal biosolids and

wastewater – Quartz Site: Phase I: Conceptual reuse design. Prepared for the Jamestown Sanitary District by Ag West Resources, 3808 Auburn Blvd. Suite 52, Sacramento, CA 95821. January 6, 1997.

Ambus, P., and S. Christensen. 1995. Spatial and seasonal nitrous oxide and methane fluxes in Danish forest-, grassland-, and agroecosystems. Journal of Environmental Quality 24:993-1001.

Bernal, S., A. Butturini, E. Nin, F. Sabater, and S. Sabater. 2003. Leaf litter dynamics and nitrous oxide emission in a Mediterranian riparian forest: Implications for soil nitrogen dynamics. Journal of Environmental Quality 32:191-197.

Blake, G.R., and K.H. Hartge. 1986. Bulk density, p. 363-375, In Methods of soil analysis Part 1: Physical and mineralogical methods, 2nd edition, ed. A. Klute, American Society of Agronomy, Madison, WI.

Braatne, J.H. 1999. Biological aspects of hybrid poplar cultivation on floodplains in Western North America: A review, USEPA Region 10, Seattle, WA.

Dumas, J.B. 1981. Sur les procédés de l'analyse organique. Annal. de Chimie XLVII. Gee, G.W., and J.W. Bauder. 1986. Particle-size analysis, p. 383-411, In Methods of soil

analysis. Physical and mineralogical methods. Part 1, 2nd edition, ed. A. Klute. Agronomy Monograph 9. ASA and SSSA, Madison, WI.

Hart, S.C., J.M. Stark, E.A. Davidson, and M.K. Firestone. 1994. Nitrogen mineralization, immobilization, and nitrification, p. 985-1018, In Methods of soil analysis Part 2: Microbiological and biochemical properties, ed. R.W. Weaver, et al. Soil Science Society of America, Madison, WI.

Horwath, W.R., and E.A. Paul. 1994. Microbial biomass, p. 753-774, In Methods of Soil Analysis Part 2: Microbiological and Biochemical Properties,ed. R.W. Weaver, et al. Soil Science Society of America, Madison, WI.

Hutchinson, G.L., and A.R. Mosier. 1981. Gas chromatographic system for precise, rapid analysis of nitrous oxide in soil. Soil Science Society of America Journal 45:311-316.


15

Menyailo, O.V., and B.A. Hungate. 2006. Tree species and moisture effects on soil sources of N2O: Quantifying contributions from nitrification and denitrification with 18O isotopes. Journal of Geophysical Research 111:1-8.

Mosier, A.R., and L. Mack. 1980. Gas chromatographic system for precise, rapid analysis of nitrous oxide in soil. Soil Science Society of America Journal 44:1121-1123.

Mosier, A.R., D.S. Schimel, D.W. Valentine, K. Bronson, and W.J. Parton. 1991. Methane and nitrous oxide fluxes in native, fertilized and cultivated grasslands. Nature 350:330-332.

Mosier, A.R., W.J. Parton, D.W. Valentine, D.S. Ojima, and D.S. Schimel. 1997. CH4 and N2O fluxes in the Colorado shortgrass steppe 2. Long-term impact of land use change. Global Biogeochem Cycles 11:29-42.

Staff, U.S.S.L. 1954. Alkaline-earth carbonates by gravimetric loss of carbon dioxide, p. 105, In L Diagnosis and improvement of saline and alkali soils, Vol. USDA Agric. Handbook 60, ed. L.A. Richards. Washington, D.C.: U.S. Government Printing Office.

Zibilske, L.M. 1994. Carbon mineralization, p. 15-40, In Methods of soil analysis Part 2: Microbiological and biochemical properties, ed .R.W. Weaver, et al. Soil Science Society of America, Madison, WI.

This research was funded by the Kearney Foundation of Soil Science: Soil Carbon and California's Terrestrial Ecosystems, 2001-2006 Mission (http://kearney.ucdavis.edu). The Kearney Foundation is an endowed research program created to encourage and support research in the fields of soil, plant nutrition, and water science within the Division of Agriculture and Natural Resources of the University of California.

assessing soil c sequestration and nutrient cycling under ...kearney.ucdavis.edu/old...

Documents