Soils with minimal long‐term organic inputs typically have reduced biological activity, which has implications for current interests in shifting from inorganic to organic inputs and promoting efficient nutrient cycling in agroecosystems. A greenhouse experiment was conducted to investigate the long‐ and short‐term effects of organic vs. inorganic N on microbial biomass, metabolic quotient, and key soil enzymes (protease, l‐histidine NH3‐lyase, and β‐glucosidase) involved in N and C cycles. Treatments applied factorially to four corn (Zea mays L.) crops grown for 306 d were: four soils from long‐term field plots (beef manure, pea vine [Pisum sativum L.], 0, or 90 kg N ha−1, each applied biennially for 59 yr); four greenhouse organic residues (pea vine, beef manure, poultry manure, or control); and four rates of inorganic N fertilizer (0–1600 mg NH4NO3‐N pot−1 with 2 kg soil pot−1). In the long‐term, soil microbial biomass and enzyme activity correlated with total C inputs. Recent organic inputs, regardless of long‐term management, had a large effect on soil biological response, which was controlled by residue composition (lignin content) and supported 80 to 400% greater microbial biomass C than the control. Long‐term inorganic N applications decreased organic matter and biological activity, whereas short‐term inorganic N applications had limited effects on soil enzyme activities and microbial biomass C, suggesting that inorganic N can maintain plant productivity during a transition to organic N sources without inhibiting the buildup of microbial biomass. The metabolic quotient gave mixed results as a soil biological indicator, being high with long‐term N or recent beef manure applications and low with recent N applications.

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Chemical and microbial aspects of soil quality are an important consideration when evaluating the benefits of soil conservation efforts such as the Conservation Reserve Program (CRP). The objective of this study was to evaluate the quality of CRP and wheat‐fallow (W‐F) soils using soil biological and chemical parameters and C and N mineralization processes. The study was conducted on 20 CRP/W‐F paired sites in eastern Washington, on Ritzville silt loam (coarse‐silty, mixed, mesic Calciorthidic Haploxerolls). Soils collected from the paired fields were analyzed for chemical and biological parameters that have been suggested as indicators of soil quality. Potential enzyme activities and soil N were higher in the CRP soil than the W‐F soil. Although there were no significant differences in total organic carbon (TOC) or microbial biomass carbon (MBC) the C mineralization potentials and C pools were significantly different between the CRP and W‐F soils. Soil biota measurements showed there was greater active bacterial biomass in the CRP soil but greater fungal‐feeding nematodes, flagellates, and amoebae in the W‐F soil. The C mineralization study suggests that there is a significant increase in the secondary C pool of the CRP soil, which may indicate a buildup of higher quality soil organic matter and the potential for higher enzyme levels. When grass or straw was added to each soil type, the W‐F soil produced more CO2 with either substrate than the CRP soil, indicating C limiting conditions in the W‐F soil. Since it is unknown what constitutes good soil quality, these shifts in chemical and biological parameters may seem subtle. However, in general, trends in the data indicated that soil quality in the CRP was improved after 4 to 7 yr, compared with its previous management in W‐F cropland.

To decrease the potential for adverse environmental impacts of N in agroecosystems, there is increasing emphasis on improving N efficiency of organic and inorganic sources. Greater reliance on organic N sources can cause short‐term crop yield decreases. Consequently, a study was conducted to determine the effect of decreasing rates of inorganic N during a transition from inorganic to organic N sources. The approach was to grow four crops of maize (Zea mays L.) during a period of 306 d in the greenhouse on soils that varied widely in organic matter and biological activity because of long‐term residue and fertilizer management. Treatments were arranged as a complete factorial with the following factors: four soils obtained from the Residue Utilization Plots (RUP), Columbia Basin Research Center, Pendleton, OR (beef manure or pea [Pisum sativum L.] vine residue, 0 or 90 kg N ha−1, applied biennially since 1931); four greenhouse amendments (pea vine, composted beef or poultry manure, or control); and four rates of N fertilizer (0–1600 mg N [2 kg]−1 soil as NH4NO3). In the absence of organic residue or N fertilizer, soil from the manure RUP produced greater dry matter yield (DMY) and plant N uptake than soil from the other RUP treatments. There was no net N mineralization from beef manure added in the greenhouse. Between 10 and 30% of the total poultry manure and pea vine N added was taken up by the four maize crops. Results showed that, by the fourth crop, when inorganic N applications had decreased to zero, plant productivity by organic residues was best maintained by pea vine residue or poultry manure.

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