How short-term Grassland Set-Asides impact soil properties in the Fraser River Delta

Introduction

In the Fraser River delta, Grassland set-asides are an important conservation practice that provide wildlife habitat and improves soil on agricultural land. A grassland set-aside in this region is defined as arable land that is taken out of production and seeded with a mixture of grass and legumes. The Delta Farmland and Wildlife Trust offers a grass land set-aside program that provides farmers with cost share payments for implementing this practice on their land for a one to four-year period. This enrolment period is shorter than typical set-aside programs which promote the establishment over five-plus years. 

Highly degraded (or unproductive) sites are commonly enrolled in both short- and long-term programs. The shorter commitment of this program has encouraged farmers in the region to rotate grass land set-aside with annual crop rotations on productive fields, or transition productive fields into organic production. 

Prior research on long-term set-asides have found this practice to improve soil structure, relieve compaction, and increase soil organic matter but only few studies have evaluated the effects of short-term grass land set-aside. Previous studies in the Fraser River delta region that have attempted to quantify the impacts of this practice on soil properties have found varying results. Since fields entering the program have extensive variability in soil conditions prior to seeding (i.e. some are productive or unproductive), it is likely that these baseline conditions are influencing vegetation and soil responses. 

Understanding the short-term benefits of this practice, and varying field responses, could potentially lead to a reduction in the number of years that farmers set land aside for, allowing more opportunities for production while also supporting ecosystem services.

The objectives of this study were to:

• Evaluate the effects of grass land set-asides on soil properties during the initial two seasons of establishment 

• Understand different vegetation and soil responses in productive and unproductive fields entering the grassland set-aside program and determine key indicators for predicting these responses

Research Process

The study was carried out on eight operational farming sites within Delta and Richmond, BC from April 2015 to September 2016. All eight study sites included a grass land set-aside field and an adjacent field managed under annual crop rotation for crops commonly grown in the region such as potatoes, beans, peas, barley, and corn. Fields were taken out of production in September 2014 and seeded in April-May 2015. 

Soil samples were collected from April 2015 to September 2016, from four randomly selected subplots per field. Prior to seeding the Grassland set-aside, soil was collected and measured for levels of sodium, carbon, and stability aggregates, as well measuring soil density and weight. ver Subsequently, soil and vegetation responses (including density, weight, porosity, and above ground growth) were evaluated over the first two seasons and compared to the paired fields continuously managed for annual crop rotations. 

Results

Two of the eight fields enrolled in the grassland set-aside program were determined to be unproductive due to a high soil exchangeable sodium content, poor soil structure, compaction and a low total soil organic carbon content. These unproductive fields did not have soil improvements in the first two seasons of grassland set-aside establishment and likely require additional accompanying management practices for improvements to occur in a short-period of time. 

An analysis was done on the remaining six fields that were deemed productive. During the second season of grassland set-aside establishment, the results demonstrated that the mean weight diameter was higher in grassland set-aside fields than paired annual crop rotation fields. Similarly, after two seasons of grassland set-aside establishment both aeration porosity and density were improved by grassland set-aside compared to annual crop rotation fields with aeration porosity being 24% greater and bulk density 7% lower in grassland set-aside.

Vegetation growth is a key contributor the wildlife habitat and soil benefits of grassland set asides. The two unproductive sites in the study had very limited vegetation growth in both years of grassland set-aside establishment, which likely contributed to the limited soil improvements observed. The study demonstrated exchangeable sodium to have a strong negative correlation to above ground vegetation in grassland set-aside fields. A further analysis was done to develop preliminary exchangeable sodium thresholds to support farmers in determining which fields to include as short term rotations in the grassland set-aside program. The analysis found an exchangeable sodium content between 0.64-2.08 cmol kg-1 to be associated with moderate vegetation establishment in the second season of grassland set-aside, while areas with a value greater than 2.08 cmol kg-1 had very low aboveground vegetation.

Implications

The study demonstrated that two-seasons of grassland set-aside rotations in productive agricultural fields could be a beneficial alternative to continuous annual crop rotation for improving soil structure, while unproductive fields likely require other accompanying practices for improvements to occur. This study also highlights the importance of considering soil baseline information when evaluating the impacts of land management practices and has set important preliminary exchangeable sodium thresholds to help producers in the Fraser River Delta determine which fields to enroll as short-term rotations in the Delta Farmland & Wildlife Trust’s grassland set-aside program.