Repeated applications of municipal wastewater biosolids is cost effective for biosolids managers, but may lead to undesirable accumulations of nutrients or contaminants. We evaluated the effects of seven years of biosolids applications on tall fescue (Festuca arundinacea Schreb.) production and nutrient availability. We compared two types of Class A biosolids applied to tall fescue on a sandy loam in western Washington. Mean annual biosolids rates of 290, 580, and 870 kg total N ha(-1) yr(-1) were compared with inorganic N and zero-N controls using a randomized complete block design. We measured yield and N uptake for each forage harvest, plant tissue metals at selected harvests, soil nitrate each fall, diethylenetriaminepentaacetic acid (DTPA)-extractable metals after five years of applications, and soil pH, available P, and organic C after seven years. Forage yields increased with biosolids rate. Apparent nitrogen recovery (ANR) for biosolids averaged 18% in 1993 (Year 1), 35% in 1994, and 46% in 1999. The ANR for inorganic N averaged 62% from 1994-1999. Residual soil nitrate was less than 25 kg ha(-1) for all treatments through 1995, but increased beginning in 1996 for the high biosolids rate. Biosolids increased soil organic C levels by 2 to 5 g kg(-1) and Bray-1 P levels by 300 to 600 mg kg(-1) (0-15 cm depth). Plant tissue Zn increased from 24 to 66 mg kg(-1) at the highest application rate. Nearly all of the DTPA-extractable metals remained in the 0- to 8-cm soil depth.

Most agricultural N O emissions are a consequence of microbial transformations of nitrogen (N) fertilizer, and mitigating increases in N O emission will depend on identifying microbial sources and variables influencing their activities. Here, using controlled microcosm and field studies, we found that synthetic N addition in any tested amount stimulated the production of N O from ammonia-oxidizing bacteria (AOB), but not archaea (AOA), from a bioenergy crop soil. The activities of these two populations were differentiated by N treatments, with abundance and activity of AOB increasing as nitrate and N O production increased. Moreover, as N O production increased, the isotopic composition of N O was consistent with an AOB source. Relative N O contributions by both populations were quantified using selective inhibitors and varying N availability. Complementary field analyses confirmed a positive correlation between N O flux and AOB abundance with N application. Collectively, our data indicate that AOB are the major N O producers, even with low N addition, and that better-metered N application, complemented by selective inhibitors, could reduce projected N O emissions from agricultural soils.

Perennial herbaceous crops such as switchgrass are important sources of cellulosic biomass for the developing bioenergy industry. Assessments of how much C will be lost or sequestered into soil and the turnover rates of that C are needed to assist producers and policymakers in determining the long‐term sustainability of biomass production. We used the natural 13C abundance of soils to calculate the quantity and turnover of C4−C inputs in irrigated fields cropped to switchgrass monocultures. Soil profile root biomass produced after three seasons averaged 3.9 Mg C ha−1 m−1 Five years of cropping showed a 1200 kg ha−1 increase in soil organic C (SOC) in the 0‐ to 15‐cm depth increment, with no change below 15 cm. The surface 15 cm of soil cropped to ‘Kanlow’ and ‘Shawnee’ had a δ13C enrichment of 3‰ above the native uncultivated soil, with 3.6‰ for ‘Cave in Rock,’ with an average 2‰ enrichment compared with the soil collected before switchgrass establishment. Enrichment in the 30‐ to 60‐ and 60‐ to 90‐cm depths averaged 1.7 and 0.9‰, respectively. The amount of soil profile C4–C determined by δ13C analysis showed a greater C input than determined by the difference in total C mass between the uncultivated native and cropped soils. The average accrual rate of C4–SOC was estimated at 1.0 Mg ha−1 yr−1 Estimates of the mean residence time of the C3−C under the irrigated C4 monocultures of switchgrass were >60 yr in the 0‐ to15‐cm and 30 to 55 yr in the 15‐ to 30‐cm depth increments. On average, 24% of SOC in the 0‐ to 15‐cm depth was derived from C4 cropping.

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