MONITORING FALL NITRATES IN THE WISCONSIN INTEGRATED CROPPING SYSTEMS TRIAL (1990-1995)

T.K. Iragavarapu [1] , J.L. Posner [2] , J.O. Baldock[2], and T.A. Mulder[2]

ABSTRACT

Minimizing the amount of inorganic N in the rooting zone following the crop harvest reduces the potential for nitrate-N (NO₃-N) losses via leaching to groundwater.  Soil NO₃-N levels in the top 3 ft. of the soil profile are being measured since 1990 in all six cropping systems of the WICST.  Among the cash grain systems, fall nitrates were highest for continuous corn and lowest in the three-phase system (CS3 – soybean-wheat frost seeded with red clover-corn) without any inorganic fertilizer-N additions.  Among the forage-based systems, fall nitrates were lowest in the low-input continuous pasture (CS6) while nitrate levels were similar between CS4 and CS.  Among the different phases, nitrate levels were highest for corn grown after plowing down alfalfa and dairy manure.  These data suggest that very careful N management during the corn phase is the key to reducing fall soil nitrate levels, and that in forage systems, there can be a large NO₃-N surplus in the soil following the corn phase.

INTRODUCTION

The amount of inorganic N remaining in the soil profile following crop harvest is an important factor that reflects the nitrate leaching potential of a particular field situation.  The two major reasons that NO₃-N accumulates after cop harvest are: 1) N uptake was less than expected (due to poor soil structure, drought, pest damage, etc.) and 2) over fertilization (due to overestimation of crop yield, failure to credit the N in manure or previous legume crops, greater mineralization than anticipated, etc.).  Since one of our objectives was to measure the environmental impact of alternative cropping systems, it was decided to monitor the soil NO₃-N levels under the six cropping systems of the WICST.

MATERIAL AND METHODS

Continuous corn (CS1) was fertilized according to the spring pre-plant nitrate-N test while the corn in CS2 received 40 lb M/acre soybean credit.  Corn in the CS3 system did not receive any inorganic N while the corn in CS4 and CS5 received 20 and 15 T/acre of dairy manure, respectively (Table 1).  Dairy manure was applied to the seeding phase and the corn phases in CS4 and C5 to equal an annual application rate of 10 T/acre (50 –cow herd with replacements on 200 tillable acres produces on average 8.1 tones of manure/acre/year – we rounded it to 10 T/a/yr).  Manure was applied at an annual rate of 10 T/acre for CS6 through 1992.  Starting in 1993, rotational grazing was started at both locations in CS6. 

In 1993, the number if cores were increased to six in groups of three, such that the three cores in a croup are taken in the row, 7.5 in. and 15 in. from the row (or equivalent in non-row crops).

In 1990, 1991, and 1992 soil samples were collected in 1 ft. increments to a depth of 3 ft. with a 1.5 in. diameter probe.  Five cores were taken per plot and composited by depth.  In eight of the 14 treatments (phases), soil samples were taken to a 2 ft. depth and nitrates in 2-3 ft. depth were predicted based on the nitrate levels in the 1-2 ft. depth.  Nitrate data from 1990-92 were used to develop regression equations to predict nitrates in the 2-3 ft. depth.

Table 1. Estimated available N in the dairy manure applied at Arlington Research Station and Lakeland Agricultural Complex in 1990-1996.^a

ARS								LAC
Cropping Year	90	91	92	93	94	95	96	90	91	92	93	94	95	96
System                                                            lb/acre
CS4	95	106	86	79	71	84	78	100	102	124	108	110	104	107
CS5	71	79	64	59	53	63	59	75	77	93	81	83	78	80
CS6	47	53	43	39	xb	x	x	50	51	62	54	x	x	X

^a First year “available N” was calculated as 40% of the total N content in the manure.  Dairy Stacked manure was applied to the seeding phase and the corn phase at a rate of 20 T/acre and 15 T/acre in CS4 and CS5 systems, respectively, and at and annual rate of 10 T/acre in CS6.

^b x = no mechanically added manure.  Paddocks grazed.

CS4 = alfalfa-alfalfa-alfalfa-corn; CS5 = oats/alfalfa-alfalfa-corn; CS6 = continuous corn

RESULTS

Nitrate levels among the cropping systems were compared on a whole-farm basis and also among the different phases of the six systems.  A whole-farm comparison was done in 1993, 1994, and 1995 when all the phases in each f the six cropping systems were present.  This comparison was made by taking the proportionate amounts of fall NO₃-N from each of the phases in a system in a particular year.  For example, in a CS2 system we assumed that one-half of the farm was in soybeans and the other half in corn.  As a result, farm-wide nitrate levels are the average of the levels under each crop.

Nitrate levels were highest in the continuous corn system (CS1) and lowest in CS6 at ARS in 1994 and 1995.  At LAC, however, nitrate levels in the forage systems (CS4, CS5, and CS6) did not differ from those in the continuous corn system possibly due to lower forage yields in these systems (Table 2). 

Among the different phases, corn is the major contributor to the NO₃-N levels in all the systems as seen in Table 2.  The surplus N balance for corn in CS4 and CS5 (Table 3) is reflected in higher amounts of fall nitrates following corn in these two systems.  Fall nitrates were consistently lower following the wheat/red clover phase of CS3.

CONCLUSIONS

The data from this trial indicate that plowed down alfalfa and manure applications could result in a build-up f fall nitrates following corn harvest, especially if recovery of N by the corn is poor due to unfavorable growing conditions.  With the exception of CS1, within a given year the differences among the systems were small.  Fall nitrate levels differed significantly among the different phases in the six cropping systems at both locations.

Table 2. Fall nitrates in the top 3 ft of the soil at the Arlington Research Station mad Lakeland Agricultural Complex sites in 1990-1995.

	Arlington Research Station                           Lakeland Agricultural Station
Year	1990	1991	1992		1993	1994	1995	1990	1991	1992	1993	1994		1995
Cropping system	---------------------------------------------- lb/acre ---------------------------------------------------
Corn:
CS1 continuous corn	87	48	91		102	160	179	198	132	86	134	129		135
CS2 corn after soybean	-1	41	105		105	172	141	-	125	78	57	81		146
CS3 corn after clover	-	-	67		83	102	144	-	-	62	59	119		130
CS4 corn after alfalfa	-	-	-		142	170	184	-	-	-	122	215		209
CS5 corn after alfalfa	-	-	101		118	156	202	-	-	81	80	183		160
Soybean:
CS2 narrow-row	78	42	74		100	85	145	55	76	86	86	79		120
CS3 wide-row	75	25	66		89	72	140	49	34	63	86	67		125
Wheat:
CS3 wheat/clover	-	26	48		65	48	85	-	49	45	56	61		100
Alfalfa:
CS4 DS alfalfa	46	32	61		80	77	117	34	67	41	78	97		118
CS4 alfalfa hay I	-	27	49		55	47	117	-	71	54	71	84		122
CS4 alfalfa hay II	-	-	60		69	46	104	-	-	63	60	91		128
CS5 alfalfa with oats	-	-	-		103	68	101	-	-	-	53	73		131
CS5 alfalfa	-	-	-		84	69	107	-	-	-	78	77		143
Pasture:
CS6	-	-	-		91	59	98	-	-	-	66	108		125

¹ staggered start – soil nitrates not tested until after first season in the rotation

Table 3.  Nitrogen balance among different crops in the Cropping Systems Trial at Arlington Research Station and Lakeland Agricultural Complex during 1991-1995

	Arlington Research Station                           Lakeland Agricultural Station
Year	1991	1992	1993	1994		1995	1991		1992			1993		1994	1995
Cropping system	---------------------------------------------------lb/acre -----------------------------------------------
Corn:
CS1 continuous corn	-1	39	98	8		4	8		57			95		-43	11
CS2 corn after soybean	-45	19	54	2		13	6		35			73		-9	34
CS3 corn after clover	-	-70	-50	-125		-110	-		-37			-41		-133	-73
CS4 corn after alfalfa	-	-	110	58		105	-		-			203		134	171
CS5 corn after alfalfa	-	81	77	-2		43	-		119			172		63	105
Soybean:
CS2 narrow-row	-188	-50	-174	-142		-206	-184		-77			-164		-219	-192
CS3 wide-row	-157	-60	-173	-143		-211	-162		-83			-105		-158	-200
Wheat:
CS3 wheat/clover	-78	-47	-91	-79		-72	-25		-24			-105		-98	-63
Alfalfa:
CS4 DS alfalfa	-31	-51	-14	-38		22	232		184			237		112	186
CS4 alfalfa hay I	-325	-190	-248	-299		-246	-202		-249			-165		-289	-279
CS4 alfalfa hay II	-	-234	-197	-222		-136	-		-234			-156		-285	-208
CS5 alfalfa with oats	46	6	96	-51		0	104		130			127		76	116
CS5 alfalfa	-328	-310	-283	-324		-310	-182		-283			-194		-268	-282
Pasture:
CS6	-137	-45	110	-71		-68	-38		101			0		-15	-18