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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-Q/A

Amendment No. 1

(Mark One)

For the quarterly period ended March 31, 2014

Or

For the transition period from                      to                     

Commission File No. 001-35334

RENTECH NITROGEN PARTNERS, L.P.

(Exact name of registrant as specified in its charter)

10877 Wilshire Boulevard, 10^th Floor

Los Angeles, California 90024

(Address of principal executive offices)

(310) 571-9800

(Registrant’s telephone number, including area code)

10877 Wilshire Boulevard, Suite 600

Los Angeles, California 90024

(Former address of principal executive offices)

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports); and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No  ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).    Yes  x    No  ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act. (Check one):

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).    Yes  ¨    No  x

As of April 30, 2014, the registrant had 38,890,048 common units outstanding.

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RENTECH NITROGEN PARTNERS, L.P.

Form 10-Q/A

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EXPLANATORY NOTE

Rentech Nitrogen Partners, L.P. (“the Partnership,” “RNP,” “we,” “us” and “our”) is filing this Amendment No. 1 on Form 10-Q/A (this “Amendment”) to its Quarterly Report on Form 10-Q for the three months ended March 31, 2014, filed with the Securities and Exchange Commission (the “SEC”) on May 12, 2014 (the “Original 10-Q”), to revise management’s conclusions regarding the Partnership’s disclosure controls and procedures. Accordingly, the Partnership hereby amends and replaces in its entirety Item 4 in the Original 10-Q.

As required by Rule 12b-15, our principal executive officer and principal financial officer are providing updated certifications. Accordingly, we hereby amend Item 6 in the Original 10-Q to reflect the filing of the new certifications.

Except as indicated above, this Amendment does not purport to reflect any information or events subsequent to the filing date of the Original 10-Q. As such, this Amendment speaks only as of the date the Original 10-Q was filed, and we have not undertaken herein to amend, supplement or update any information contained in the Original 10-Q to give effect to any subsequent events. Accordingly, this Amendment should be read in conjunction with the Original 10-Q and any documents filed by us with the SEC subsequent to the Original 10-Q, including our Quarterly Reports on Form 10-Q for the quarters ended June 30, 2014 and September 30, 2014.

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PART I. FINANCIAL INFORMATION

Evaluation of Disclosure Controls and Procedures. We maintain disclosure controls and procedures, or DCP, that are designed to ensure that information required to be disclosed by us in reports that we file or submit under the Securities Exchange Act of 1934, as amended, or the Exchange Act, is recorded, processed, summarized and reported within the time periods specified in the SEC’s rules and forms, and that such information is accumulated and communicated to our management, including our general partners’ principal executive officer, or Chief Executive Officer, and principal financial officer, or Chief Financial Officer, as appropriate, to allow timely decisions regarding required disclosure. In designing and evaluating DCP, management recognizes that any controls and procedures, no matter how well designed and operated, can provide only reasonable assurance of achieving the desired control objectives. Management is required to apply its judgment in evaluating the cost-benefit relationship of possible controls and procedures.

In our Quarterly Report on Form 10-Q for the quarter ended March 31, 2014, which was originally filed on May 12, 2014, our Chief Executive Officer and Chief Financial Officer concluded that our DCP were effective as of March 31, 2014. Subsequent to that evaluation, management identified the material weaknesses in internal control over financial reporting, or ICFR, described below, and our Chief Executive Officer and Chief Financial Officer concluded that our DCP were not effective as of March 31, 2014.

Material Weaknesses in Internal Control over Financial Reporting. We identified the following material weaknesses that existed at March 31, 2014. A material weakness is a deficiency, or combination of deficiencies, in ICFR, such that there is a reasonable possibility that a material misstatement of the Partnership’s annual or interim financial statements will not be prevented or detected on a timely basis.

We did not design and maintain effective internal controls over (i) the review of the cash flow forecasts used in the accounting for goodwill, (ii) the determination of the goodwill impairment charge in accordance with generally accepted accounting principles, and (iii) maintaining documentation supporting management’s review of events and changes in circumstances that indicate it is more likely than not that a goodwill impairment has occurred between annual impairment tests. Specifically, with respect to (i) and (ii), we did not design and maintain effective internal controls related to determining the carrying value and fair value of reporting units for the purpose of performing goodwill impairment testing, documenting management’s review of assumptions used in the forecasts, and verifying that data contained in reports provided by specialists reconcile to the information provided to those specialists.

These material weaknesses did not result in a material misstatement to the Partnership’s consolidated financial statements for the quarter ended March 31, 2014. However, these material weaknesses, if unremediated, could, in a future reporting period, result in a material misstatement to the annual or interim consolidated financial statements that would not be prevented or detected by the controls.

Plan for Remediation of the Material Weaknesses

We have implemented and are continuing to implement a number of measures to address the material weaknesses identified. Specifically, we are designing additional controls over documentation and review of the inputs and results of our cash flow forecasts, the use of the work of specialists and the identification of events and changes in circumstances that may indicate potential impairment of goodwill. These controls are expected to include the implementation of additional review activities by qualified personnel and additional documentation and support of conclusions with regard to accounting for goodwill impairment calculations. We are also designing additional controls around identification and documentation relating to accounting for goodwill impairment. These controls are expected to include the implementation of additional review activities by qualified personnel, additional documentation and the development and use of checklists and procedures related to accounting for goodwill impairment calculations. We are in the process of implementing our remediation plan, and expect the control weaknesses to be remediated in the coming reporting periods. However, we are unable at this time to estimate when the remediation will be completed.

The process of designing and implementing an effective financial reporting system is a continuous effort that requires us to anticipate and react to changes in our business and the economic and regulatory environments and to expend significant resources to maintain a financial reporting system that is adequate to satisfy our reporting obligations. As we continue to evaluate and take actions to improve our ICFR, we may determine to take additional actions to address control deficiencies or determine to modify certain of the remediation measures described above. We cannot assure you that the measures we have taken to date, or any measures we may take in the future, will be sufficient to remediate the material weaknesses we have identified or avoid potential future material weaknesses.

Changes in Internal Control Over Financial Reporting

There were no significant changes in our ICFR during the quarter ended March 31, 2014 that materially affected, or are reasonably likely to materially affect, our ICFR.

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PART II. OTHER INFORMATION

Exhibit Index

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized.

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