The Effects of Field Crop Ecology on C and N Pools in Sustainable System

Ezanno, A.M., R.R. Harwood, E.A. Paul, J.T.Ritchie, N.E. Ostrom

Presented at the All Scientist Meeting (1998-07-21 to 1998-07-22 )

The Effects of Field Crop Ecology on C and N Pools in Sustainable SystemConventional farming practices can result in significant C and N loss from the soil. The use of organic amendments in place of N fertilizer can provide crop nutrient requirements, build C and N stores, and improve soil quality parameters. The implementation of sustainable agriculture techniques should cause C an N to cycle differently than in a conventional cropping system. Ideally slow and active pools of N in particular would increase in size and respond more rapidly to environmental stress. Sustainable agriculture management practices that include: rotations, cover crops, and compost change the flow of C and N in the active and slow soil pools. Fluctuations in C and N pools affect the amount of plant available N in a cropping system. Conditions under grasslands are ideal for C and N storage. Slow pools of N and C are larger and the grassland system is more rapid in responding to decreases in inorganic N as compared to conventional cropping systems.Field sites will be located at Kellogg Biological Station in Hickory Corners, MI. All cropping treatments will be located on the Living Field Lab. The LFL has a factorial design that allows for the comparison of a number of interactions. These interactions include: differences in management practices (Integrated-Fertilizer vs. Integrated-Compost), rotation effects, and cover crops. The rotation consists of the following crops in the order listed, corn (Zea mays L.)/ corn,/soybean (Glycine max)/wheat (Triticum aestivum). Cover crops vary depending upon the point in the rotation. Crimson clover (Trifolium incarnatum L.) is seeded to 1st yr. corn, 2nd yr. corn is in rye grass (lolium multiflorum), soybeans have no cover, and red clover (Trifolium pratense L.) is seeded into wheat plots.The LFL does not contain a fallow treatment or grassland. Therefore, a grassland treatment on the Long-Term Ecological Research (LTER) plots will be compared to the LFL cropping systems. Four replicates of the historically tilled, successional grassland treatment will be incorporated into our research. The replicates that will be sampled on the LTER are: T7 r2, T7 r3, T7 r5, and T7 r6. Replicates 1 and 4 were not chosen due to the large amount of woody vegetation present. The LTER statistical design is a randomized complete block.This study will focus on the initial release of inorganic N from plant residues and compost, the incorporation of residue and compost materials into the active and slow pools of SOM, and subsequent release of plant available inorganic N from the active and slow pools. The effect each point in the crop rotation has on potentially mineralizable N will be assessed separately and in conjunction with cover crops and/or compost. The release of N from plant residues and subsequent incorporation of residues into SOM will be evaluated using an in situ buried bag experiment. Estimates of rate constants and sizes of active and slow pools of soil C and N will be estimated from long term incubations. The flow of C from corn and N from clover cover crops will be traced through particulate organic matter fraction (POM) using isotopic signatures, 13C and 15N. Modeling with SALUS will determine the length of time required for C and N pools in each system to reach a steady state. Our research will provide growers with information on the quantity of potentially mineralizable N at various points in the rotation and the impact compost and/or cover crops have on potentially mineralizable N. This additional knowledge of C and N cycling will allow farmers to manage: rotations, compost applications, and cover crops in a manner in which plant available N can be optimized at the appropriate time for a given crop. Modeling will allow us to simulate future effects of rotations and inputs. As pools of C and N in the systems reach a steady state, changes in farm management practices that may be required can be addressed prior to implementation.Previous LFL Soil Quality Research.  Prior scientific investigators emphasized changes in N mineralization rates over a growing season as affected by crop rotation, cover crop, and compost (Willson et al., In press). It was determined that N mineralization fit a seasonal pattern (Fig. 1). Kinetics of the active N pool in compost plots were found to be significantly different from those of N fertilizer treatments. In the 1st yr of compost application mineralization rates increased 23%. Three years of compost additions resulted in: a 12% increase in the pool of mineralizable N, a 27% decrease in MRT, and a 28% reduction in inorganic N without decreased yields. Treatments that contained legume cover crops manifested short-term increases in potential mineralization rates and a slight decrease in MRT. Thus, the data suggest that the use of compost may increase C and N mineralization across the entire season (as opposed to several weeks) and maximize storage of soil C and N. This may increase active and slow pools of C and N, as well as, the rate of N and C cycling between active and slow pools.On-Going Soil Quality Research Related to the LTER.  Previous research on the LFL has suggested that active soil N pools have increased in compost treatments. The rate of N cycling has been augmented. These increases should also be reflected in C pools. The completion of two rotation cycles may reveal similar changes in N and C pools due to crop ecology and cover crop use. A basic premise of sustainable agriculture is that inputs can be decreased and nutrient cycling increased without decreasing yields. The use of crop ecology and N sources other than N fertilizer are expected to facilitate C and N cycling between active and slow pools. Unmanaged systems such as successional grasslands are ideal for C and N storage. Slow pools of N and C are larger and the grassland system responds more rapidly to decreases in inorganic N as compared to conventional cropping systems. Thus, there is a potential agronomic benefit to applying ecological principles to agriculture.Carbon and N mineralization from 150d incubations on several agronomic treatments will be compared to treatment 7 of the LTER’s, previously tilled-successional grassland. Treatment 7 will be used as a base line for comparison of shifts in cropping treatments. Grassland treatments tend to have greater C and N mineralization potential during laboratory incubations. A decomposition/POM fraction study has been initiated on treatment 7 of the LTER and several LFL treatments. Crop residues from the LFL will be buried in LTER plots. The decomposition and incorporation of crop residues into POM on the successional grassland will be contrasted with data from agronomic treatments on the LFL.Return to Contents