Basic Plant and Soil Process Measurements for Range Ecosystem Modeling and Management

Xuejun Dong, Paul Nyren, Bob Patton, Brian Kreft and Anne Nyren

NDSU Central Grasslands Research Extension Center


 

Table of Contents

Introduction

A new chamber for measuring canopy photosynthesis in the field

Measurements useful for studying grassland water dynamics Introduction

Measurements useful for studying grassland carbon/nitrogen dynamics



Introduction


During the growing season of 2004, two new measurements to characterize the function of the rangeland soil-plant system under cattle grazing were completed. First, a chamber to measure CO2 exchange rate from the grass canopy, including soil respiration rate was built and utilized. Second, seasonal changes in soil microbial biomass as well as soil mineral nitrogen content were measured in the same grazing intensity study site. In the following sections, these measurements will be described in more detail in relation to the grassland modeling project.



A new chamber for measuring canopy photosynthesis in the field


In past years difficulty in measuring gas exchange was experienced because grass leaves were often too short (especially under grazing) to be enclosed in the equipment chamber. A plexiglass chamber was constructed to overcome this problem. This chamber is 24 inches tall, covers a ground area of 18 X 18 inches and is transparent to allow maximum light transmission (Figure 1). It was designed to be portable with various accessories and wires detachable for easy assembly and transportation. The main accessories are:

 

1. A blower used to force free air to flow through the system at an appropriate speed to allow a sufficient gas exchange between the enclosed soil-plant system and the moving air. The airflow prevents excess temperature increase inside the chamber.

2. A volume flow meter to monitor air flow rate through the system.

3. Two mini-fans used to provide appropriate mixing of the air inside the chamber.

4. Light and temperature sensors installed to the chamber wall for environmental monitoring.


Eighteen metal frames to hold the chamber tightly during measurements were permanently inserted into the ground to allow repeated sampling over the growing season. This new chamber used in combination with the LI-6400 photosynthesis meter has not only supported the current research project, it will also be used to provide data for the study of grassland carbon sequestration in the future.

 


Measurements useful for studying grassland water dynamics

 

The major field and laboratory measurements required for modeling grassland water dynamics have also been conducted. These include soil physical properties (bulk density, saturated hydraulic conductivity, and soil water characteristic curves) and plant-water relation data (leaf water potentials, leaf pressure-volume curves, stomatal conductance) and major environmental variables associated with the leaf conductance measurements.


Stomatal conductance (ks) is influenced mainly by several environmental factors, including photosynthetic photon flux density (Qp), temperature (T), vapor pressure difference (de), and ambient CO2 concentration (Ca), as well leaf water potential (Yl). A simple approach is to use a multiplicative function, ks (Qp, T, de, Ca, Yl) = ks (Qp) ks (T) ks (de) ks (Ca) ks (Yl).


The exact nature of each of these functions was determined in controlled experiments in the past decades. We will use field data to estimate optimal parameter values for each of the 5 functions using the least-squares approximation method. The aim is to find simple equations for describing variations in stomatal conductance as observed in plants in the Missouri Coteau Grasslands of North Dakota.



Measurements useful for studying grassland carbon/nitrogen dynamics

 

(1). Soil respiration (Rs) as a function of soil temperature (Ts) and water content (qs). There has been worldwide interest in this topic in the past several years. These results have characterized the basic nature of the function Rs = Rs (Ts, qs). Several reports considering the effects of animal grazing on soil respiration have been made. But the following question is not answered in a quantitative way: how and to what extent does cattle grazing shift the Ts and qs dependences of Rs? From May to November 2004, we measured soil respiration for about 35 days (about 5 days a month), with measurements made at different times throughout the day (10 am-1pm, 2pm-5pm and 10pm-12pm local time). This intensive measurement will be used to answer the above question.


(2). Soil microbial biomass and mineral nitrogen content. A central mechanism of soil carbon and nitrogen cycling involves the amount and activity of microorganisms in soils. In the 2004 growing season, this variable was measured three times during the growing season. The microbial carbon/nitrogen ratio will be determined. Soil soluble nitrogen (ammonium and nitrate) contents during the growing season was also measured. This datawill be used to evaluate the variability of total soil microbial biomass and activity under different grazing treatments (non-grazing, moderate and heavy grazing).


(3). Using the newly built chamber (Figure1), canopy photosynthesis was measured in relation to grazing and drought from May through September of 2004. This is the second year of drought treatment using rainout shelters. The data, as well as that collected in 2003, will be combined to address the question of the impacts of grazing and drought on rangeland photosynthetic capacity.

 

In 2005, two major measurements will be made. First, the monthly measurements of rangeland CO2 exchange (photosynthesis and respiration) will continue, beginning from April to November. After 2-3 years of data collection, we hope to provide an estimate of the annual carbon budget in the Missouri Coteau grasslands. The plexiglass chamber together with a set of portable rainout shelters designed at the CGREC allows us to measure the carbon flux in small field plots with particular drought treatments. This cannot be done using the typical meterological method of measuring CO2 flux. Secondly, beginning the winter of 2004 , a greenhouse experiment will be conducted to measure nitrogen uptake kinetics for selected range grasses. The data will be used to estimate the major parameters of the nitrogen uptake equation used for the modeling of grassland carbon/nitrogen dynamics.


NDSU Central Grasslands Research Extension Center
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