Assessment of long-term management impact on soil C dynamics in subtropical grasslands

Assessment of long-term management impact on soil C dynamics in subtropical grasslands

Summary

Long-term total soil organic carbon (SOC), total nitrogen (TN), total C:N ratio (C:N), root biomass C and above-ground biomass C was assessed under 3 grassland management systems (intensively managed improved pasture – Pasture, Slash Pine-Bahia silvopasture – Silvopasture, and native rangelands – Rangeland). Results suggest that improved management is beneficial for soil C sequestration especially when native rangelands are converted to more intensive production systems. However, integrated management systems, such as silvopastures, does not seem to offer higher soil C sequestration benefit compared to intensive improved pastures. Density fraction assessment of the soil C and assessment of CO2 efflux are still underway.

Objectives/Performance Targets

i. To quantify soil organic C stocks and particulate organic C under 3 grazing land systems subject to different management intensity levels.
ii. To assess for differences in the contribution of above- and below-ground components under the 3 distinct grazing land management systems.
iii. To determine the long-term rate of C loss in form of CO2 efflux across seasons under each management system. This effort is expected to foster a deeper knowledge on the contribution of autotrophic and heterotrophic respiration to soil C losses under the distinct management systems.

Accomplishments/Milestones

Objective 1: In 2012, 5 sampling quadrats (20 × 20m) were demarcated along a diagonal transect within each replicate field (2 replicate per management system), and a total of 450 random soil cores (5 samples × 3depths × 5 quadrats × 2 replicate fields × 3 management systems) were collected, processed and analyzed for total C and N concentrations. The POM analysis was performed with slight modification to the method presented by Cambardella and Elliot (1992). Additional isotopic analysis on the bulk soil and POM fraction is currently underway and is expected to provide important information relative to the source of long-term changes in soil C.
Soil organic C: SOC stocks under the three management systems was not significantly different at 0-10cm soil profile depth (p=0.110), but significant differences were observed at 10-20cm and 20-30cm depths (p=0.005, p=0.022, respectively). Cumulative SOC, across the entire sampled soil profile (0-30cm) was also affected by grassland management (p=0.008). Soil organic C accumulation was promoted by management intensification. Similar SOC stocks (0 to 30 cm depth) were observed in the Pasture and SR field, while the lowest SOC values were obtained under the Rangeland fields. At lower depth interval (20-30cm), the soil organic C sequestered under Rangeland (7.32 Mg C ha-1) and Pasture (11.13 Mg C ha-1) was statistically similar, but Silvopasture showed significantly higher SOC compared to Rangeland. Also, SOC stocks declined with soil depth, ranging from a high of 26.53 Mg C ha-1 in Pasture fields (at 0-10cm depth) to a low of 7.32 Mg C ha-1 in Rangeland fields (at 20-30cm depth).
Soil N: Similarly to SOC responses, soil N stocks were affected by grassland management systems. Soil N stocks (0 – 30 cm depth) in Pasture and Silvopasture were not significantly different, despite the difference in the quantity of N fertilizer applied in the two management systems. Across all depths, soil N in Rangeland was consistently lower in comparison to Pastures, while it was only lower at 0-10cm depth in comparison to Silvopasture.. The highest N-content at 0-10cm (1.71 Mg ha-1) was observed in Pasture, while Silvopasture contained highest N content at other soil profile depths, and across the entire soil profile depth (0-30cm).
Soil C:N ratio: The bulk soil mean C:N ratio ranged between 15 to 22. Mean C:N value of Rangeland fields was significantly higher in comparison to Pasture and Silvopasture at 0-10cm soil profile depth. However, the mean C:N ratio across the sampled soil profile depth (0-30cm) indicate that there is no significant difference between Rangeland and Pasture, and between Pasture and Silvopasture. Furthermore, there was no significant difference between the ecosystems at 10-20 and 20-30cm soil profile depth intervals. The similarity of soil C:N in Pasture and Silvopasture across all depths is attributable to the similarity of their SOC and soil N content.
Objective 2: To assess the root biomass, 3 random soil cores (5cm) were sampled from each previously demarcated quadrat at three depth intervals (0-10, 10-20, 20-30cm). The root biomass was processed by drying and determining the ash content.. To quantity the above-ground biomass C, double-sampling methods were applied on the Pasture and Rangeland fields, while a combination of double-sampling method and application of allometric equations (based on measured tree DBH) were applied for the Silvopasture fields.
Root Biomass: Root biomass varied between the management systems and was significantly different at soil depths of 0-10cm and 10-20cm. At the 0-10cm depth, Rangeland contained greater root biomass (12.30 Mg ha-1) compared to Pasture (7.25 Mg ha-1) and Silvopasture (4.76 Mg ha-1). However, at the 10-20cm depth, root biomass C is highest in Silvopasture (13.59 Mg ha-1), lower in Rangeland (6.22Mg ha-1), and lowest in Pasture (1.66 Mg ha-1). Across the entire soil profile (0-30cm), root biomass of the Rangeland and Silvopasture fields were not significantly different (21.70 Mg ha-1, and 21.74 Mg ha-1 respectively), while it was significantly lower in Pasture fields (11.22 Mg ha-1). Within Rangeland and Pasture, the highest proportion of the total root biomass C (57% and 65%, respectively) accrues to the 0-10 cm depth, while it accrues to the 10-20cm depth (63%) in Silvopasture.
Aboveground Biomass C: The total aboveground biomass was greater in the Silvopasture fields (70Mg C ha-1) compared to Rangeland and Pasture (4.18Mg C ha-1 and 2.07Mg C ha-1, respectively). Woody vegetation components of the Rangeland and tree components of the Silvopasture account for 70% and 98% of their total aboveground vegetation biomass, respectively. The double sampling methods adopted for biomass quantification yielded acceptable R2 values (0.69 – 0.87) in relating biomass to related and more frequently measured parameter (specifically, vegetation height and percentage coverage).
Objective 3: In determining the long-term rate of C loss from the different management systems, we deemed it important to provide a much needed understanding of the relative contributions of heterotrophic and autotrophic respiration to soil C losses under these management systems. The root and microbial respiration were partitioned with fabricated 30cm3 exclusion boxes. Each fabricated exclusion box were installed by carefully digging around a 30cm2 rectangular area to severe the roots up to 30cm soil depth where over 80% of roots are concentrated, with minimal disturbance to the formed soil column. The boxes leveled with the soil surface and effectively cut off the roots within the 30cm3 soil column. The vegetation within the boxes was periodically clipped to ground level in order to prevent photosynthetic production and avoid allocation of carbohydrate to the roots from the aboveground production. For the SCO2 measurement routine, two (2) PVC collars (height = 5cm) were installed on the soil surface at each measurement location to ensure snugly fit of the portable CO2 analyzer chamber during measurement, and avoid potential errors from possible CO2 leakages. PVCM was installed within the exclusion box and designated for measurement of microbial (heterotrophic) respiration. Since the vegetation is non-existent and the roots are dead (or/and inert), the CO2 efflux from this PVCM collar is attributable to microbial organisms who are thriving on the available and accessible stored carbon within the soil column. PVCRM was installed at ~40cm away from each exclusion box to capture the total soil (root + microbial) respiration. The vegetation within the PVCRM collar was consistently clipped to ground level before each measurement to avoid any influence of shoot respiration. The contribution from root respiration will be eventually calculated as the difference between CO2 efflux from PVCRM and PVCM.
The measurement procedure started in January, 2013 and will continue till Dec. 2013, hence, results are currently not available to draw clear inference relative to the research objective.

Impacts and Contributions/Outcomes

Results from this research have been disseminated at professional meetings (Southern Association of Agricultural Scientists (SAAS) conference (Agronomy Session), held in Orlando, FL) and extension materials. This research effort is expected to stimulate further scientific inquiry into long-term C sequestration under mainstream grassland productions systems (with a “system” perspective). This research will benefit producers by providing scientific information of the potential gains and losses of carbon in response to grazing land intensification. In addition, this effort will be important for subsequent economic analysis of the trade-offs between intensive/improved production systems and integrated systems. We will disseminate our findings through extension activities (such as our research station’s annual field day) and through the widely-read newsletter of the Florida Cattlemen Association. This ongoing research effort will provide timely information for livestock producers about appropriate management decisions that will favor livestock and forage production as well as enhance their opportunity for participation or future involvement in carbon credits trade. Furthermore, research results will be published in a refereed journal for wide public access, and a professional presentation will be made at the upcoming global soil C conference organized by the International Union of Soil Scientists (IUSS).

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