Is Residue Management an Important Factor in the Soil Health of Perennial Grass Seed Production Systems?

B. Verhoeven, A. Moore, D. Sullivan, M. Gonzalez-Mateu
Oregon State University
Introduction
The aim of this work was to determine if straw management and stand age affects soil health outcomes in tall fescue seed production. In addition we looked at the effect of soil clay content on soil health indicators. To accomplish this, fourteen paired sets of fields were sampled (28 fields), representing tall fescue fields with either a history of full straw chop back (“full straw”) or continuous straw removal (“baled”). Returning post-harvest residues to the field is one method of achieving higher OM inputs to a system and hopefully improve soil health. Prior to the 1990's, most tall fescue fields were opened burned followinng harvest. With the phase out of field burning, the vast majority of grower have moved to baling and removing straw after harvest. The straw is a relatively low quality organic matter, but does contain around 100 lbs/a of potassium (K) and on average around 2175 lbs/a of carbon (C) (5000 lb/a biomass x 43.5% C). Growers are aware of the need to replace K with potash fertilizer, but the effects of removing the C and OM on over all soil health properties have not been studied in these systems. Data is needed to help better inform producers of possible trade-offs associated with long-term straw removal. Data needs to be collected in a way that allows us to begin differentiating the effects of management versus soil type on measures of soil health.
Methods
Study Design:
  • Survey based study in perennial tall fescue grass seed fields in western Oregon
  • Fields all greater than 4 years in age to reduce impacts of tillage
  • Paired fields based on: geographically close, similar soil texture (< 5% difference in clay content), close in field age
Figure 2. Map of the Willamette Valley showing the general location of the 14 sites (one full straw and one baled field at each). 
 
Soil health indicators and sampling methods:
  • Three transects sampled within each field
  • Indicators and methods based on Cornell Comprehensive Assessment of Soil Health
  • Linear mixed effects model and regression analysis, clay used as co-variate
Table 1. Soil health parameters measured in this study
Chemical/nutrient
Physical
Biologic
pH
Bulk density
Soil respiration (24 and 96hr)
Electrical conductivity (EC)
Wet aggregate stability (WSA)
Total C%, total N%
Mehlich-3 extractable P,K,Ca,Mg
 
Active C (permanganate oxidizable C)
Cation exchange capacity (CEC)
 
Potentially mineralizable carbon (PMN)
 
 
Results: effects of soil clay content and field age
  • Soil clay content ranged from 15.4% to 47.8%, with a mean of 25.7%
  • Clay content had a significant effect on most measured soil health indicators, the only measures not affected were soil test K, pH, bulk density, and potentailly mineralizable nitrogen. 
  • Soil C and N and water stable aggregates increased with clay content under both straw management practices
  • Permanganate oxidizable carbon, respiration rates, and soil test Ca all increased with clay content in full straw fields only
  • Soil test P declined with clay content in baled fields only 
 Figure 3.  Relationships between soil clay content and select soil health properties for full straw (salmon) and baled (blue) in 0-8". Each point represents a field and is the average of the three transects. A regression line is shown only when the regression was significant at p<0.05.
 
  • Field age ranged from 3 to 18 years, with a mean of 7.6 years
  • Soil C and N increased with field age, and increased more strongly with age in teh full fields
  • No other soil health indicators were affected by stand age
Figure 4. Relationships between stand age and select soil health properties for full straw (salmon) and baled (blue).  Each point represents a field and is the average of the three transects. A regression line is shown only when the regression was significant at p<0.05.
Objectives
  1. Evaluate soil health measurements under bale versus full straw chop-back management practices in tall fescue seed crops
  2. Explore relationships between soil health measures and key soil/site properties (i.e. texture) in tall fescue seed cropsFigure 1. Example of loading harvested straw bales onto a trailer. Photo credit: Steve VanMouwerik, Anderson Hay and Grain Co. https://forages.oregonstate.edu/tallfescuemonograph/endophyte_seed_straw/management
Results: straw management
  • No effect of straw management on bulk density or wet aggregate stability
  • No effect of straw management on soil carbon or permanganate oxidizable carbon (active carbon)
  • Significantly higher soil test K, elevated soil test P in full straw fields
  • Elevated respiration in full straw fields Figure 2. Box plots of key soil health properties in the baled and full straw fields (n = 14).  Each point represents a field and is the average of the three transects. The top of each box represents the 25th percentile, while the lower end of the box represents the 75th percentile (i.e. 25% of observations were above and below the boxed area). Solid bold lines indicate the mean for each management, respectively. 
Conclusions
  • Our data shows that soil clay content was a powerful driver of many soil health outcomes and should be taken into account when analyzing soil health data from Willamette Valley soils.
  • When interactions between soil clay content, stand age and residue are considered, we saw that total carbon, microbial activity and nutrient retention increased in fields with full straw loads.
  • Soil total C and N increased with stand age regardless of straw management, indicating that as the time since disturbance increases, these properties are likely to increase. 
  • Both soil clay content and stand age were stronger driver of soil carbon than straw management.
  • This project demonstrated mostly positive but quite modest changes in soil health in response to retaining grass seed residues in the field.
  • This work provides baseline data on the range range in soil health properties present in contemporary soils used for grass seed production in the Willamette Valley
 

Abstract

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