Evaluation of soil fertility and soil quality aspects of organic perennial pastures and annual forage crops.

摘要

The specific objectives of the study were to (i) estimate N mineralization (Nmin) from a soil receiving CDM and subsequent N uptake by organically grown perennial forage grasses, (ii) to quantify Nmin from a soil receiving CDM and RDM amendments and subsequent N uptake by organically grown annual forage crop, (iii) to evaluate how RDM and CDM amendments managed for organic annual forage crops and perennial pasture grasses impact the abundance, biomass and species-composition of earthworm communities in the short-term following implementation of management practices, and (iv) to evaluate the short-term impact of variable rate applications of CDM on soil microbial biomass carbon (MBC) and nitrogen (MBN) as well as aggregate stability under a perennial grass system.;To accomplish objective (i), a field incubation study was conducted at the perennial study site using an in situ intact core resin bag technique to estimate Nmin under two perennial grass mixtures: (1) orchardgrass-smooth brome-meadow brome and (2) hybrid wheatgrass-tall fescue-hybrid brome. CDM was surface broadcast at 22.4 and 11.2 Mg ha -1 (wet weight) in 2008 and 2009, respectively. To accomplish objective (ii), a field incubation study was carried out at the annual study site and Nmin was determined as described in objective (i). For this study, the application rates of CDM and RDM were designed to achieve approximately 123 and 56 kg total N ha-1 in 2008 and 2009, respectively. To accomplish the first part of objective (iii), earthworms were sampled from the annual study site in July 2009. To achieve the second part of objective (iii) and objective (iv), variable rate CDM applications of 0, 22.4, 33.6, and 44.8 Mg ha-1 (wet weight) were topdressed onto a grass mixture consisting of orchardgrass, smooth brome, and meadow brome. Earthworms and soil samples were collected from the perennial study site in July of 2009.;For Nmin at the perennial site, results indicated that regardless of the grass mixture net soil Nmin was higher in 2008 compared to 2009, even though soils were relatively drier and cooler. Overall, this study indicated that CDM applied at the rates used in the present study might not meet the N needs of perennial forage grasses. Thus, higher quality CDM than the one used in the current study or alternatively other sources of N might be needed for optimum forage yield.;For Nmin at the annual site, there was a significant ( p0.05) difference in mineralized N between RDM and CDM amendments in 2008 in plots seeded with teff; however, there was no significant N mineralization response to CDM and RDM amendments under bare-fallow plots. Overall, this study suggests that RDM can provide sufficient in-season plant available N to support high dry matter yields of annual forage crops such as teff, particularly in the year following application.;The earthworms identified from both the annual and perennial studies were composed entirely of the endogeic species Aporrectodea rosea (Savigny), A. tuberculata (Eisen), and A. turgida (Eisen), the first being found only in the perennial pasture. Regarding the annual study, RDM and CDM application did not influence earthworm abundance and biomass in teff plots. This study highlighted that endogeic earthworms can be negatively affected at concentrations often considered non-saline for most plants.;The study looking at soil microbial biomass and aggregate stability found MBC and MBN to be significantly (p0.05) higher at the CDM application of 44.8 Mg ha-1 than at the other rates except for the 0 Mg ha-1 treatment of no CDM application, where alfalfa was interseeded into the perennial grass mixture. This trend was also reflected by increased aggregate stability at the 44.8 Mg ha-1 CDM application rate as compared to the other rates considered in this study. (Abstract shortened by UMI.)